Otherwise you have to take the pulley off and mount the pump, then install the pulley at least on a Commander 351 that  I'm familiar with, not familiar with the bracket setup on your 302 Commander, but it may be the same  or maybe this is your 302 Crusader, so it might be different.

Some altering of the spacing of the bracket from the engine may be necessary too, for belt alignment.

So, it could take some "rigging" but should be doable.

If you're thinking about a new Sherwood, you should at least think about a cheap Korean knockoff from the DJ pump company that's more corrosion resistant because the bearing housing is made of bronze instead of cast iron like the Sherwood.. All the other parts interchange

Here's a link to the pump on EBAY, I've bought a couple from the outfit in the link and they've worked well, with about 100 hours on each pump this past summer and no issues..

http://www.ebay.com/itm/Sherwood-G21-G20-GT-40-Raw-Water-Sea-Pump-5-0-5-8-302-351-PCM-Pleasurecraft-FORD/264662868185?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3D1f81a6a915484772b6de45d2f26793b4%26pid%3D100675%26rk%3D2%26rkt%3D15%26mehot%3Dag%26sd%3D133349223107%26itm%3D264662868185%26pmt%3D0%26noa%3D1%26pg%3D2380057&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3Ae6bccbdd-8b71-11eb-9f0e-9672f398f689%7Cparentrq%3A5c90abbd1780acb1951bd03bffd0e624%7Ciid%3A1" rel="nofollow - link

OR is this thing dead.  Pics below.  Please give me an opinion.

Just to save you some time, I'll say it, but I'm a backyard hack who would have no problem trying it just to see what would happen  

You'll probably have to figure out what the plastic is and what will bond to it.

I'd guess it's a phenolic type material like carburetor spacers are made of.

I'd probably get the trusty Swiss Army Knife out to get rid of the rubber.

It looks like a bit of a "rig" to begin with since that looks like some clear plastic tubing on one of the ports so you could use 1 inch hose on one side and 1/1/4 inch hose on the other.(old backyard hack maneuver)

What do you have to lose besides a little time, effort and cooling water?

Or you could do a Google search for Jabsco 17410-0000 and find a place that has one in stock, there seem to be some out there. Lots of places say "out of stock" but not all of them.

Disclaimer................ I never claimed to be smart or anything even remotely close 

It looks like  this goes to your 78 Commander from the above pictures

A smaller sheave will speed up the pump RPM's. How this affects the cooling is tough to answer. You would need to look at the original pump RPM/GPM specs and compare it to the replacement pump RPM/GPM specs. I would just try the 6" and see what happens. Typically our RWP systems are more than adequate and dump excess out the exhaust. 

Why the 6"? Clearance?

They're a pretty tight press fit on the later Sherwood pumps. Older ones like in the picture had a keyway, key and set screw.

Going back on was a case of heating the pulley for a while and slipping it on the shaft, pretty much like installing a coupler on a straight prop shaft, but somewhat less heat.

4th post down in the link below fanofccfan tells how he did his replacement  

http://www.correctcraftfan.com/forum/forum_posts.asp?TID=42829&title=jabsco-17410-replacement" rel="nofollow - link

Here's a picture of the bracket that encircles the pump, maybe yours is different

First: Cut our RWP bracket and drilled a new hole to accept an older Sherwood that we had on the parts table.  Popped in a new impeller and gasket and we are back!  Ran it and temp was nice.  Yes, a little backyard hackery on the bracket but oh well.   

Second: We created an issue when we took the old RWP off.  We sheared a bolt.  Pics show which one as it is missing.  Hadn't mentioned it bc we thought we would use an extractor to get out the ~1" that sheared off.  We don't have a long enough extractor though.  I will lead off with the fact that we want to fix this right and aren't running the boat other than our test until fixed.  This bolt seems to be important as it seals the housing.  It also provides extra support for the RWP/RWP bracket to reduce vibration.  Any ideas on getting it out?  It is deep.  Our other thought is to drill and tap a new shallower bolt hole.  The engine/block looks to have enough meat to move to a 7/16 or 1/2.  

If so here is what your up against - that bolt broke off in the block and you have the thickness of the timing cover and about 5/16" of the circulating water pump. Your probably going to have to pull the front of the engine off- dampner,water pump,bolts that go into the timing cover on the front edge of the oil pan,timing cover etc and go from there.

If it's the long bolt that Gary mentions, just convince yourself that you need a new timing set and when you're doing that job you'll have unobstructed access to the broken bolt when the timing cover is off. 

if you need any help in the "convincing" department, with a 78 engine, unless the timing set has been replaced sometime in the past, you have a good chance of having a cam gear that's aluminum with nylon coating on the teeth and it eventually starts losing the nylon, makes the timing erratic and leaves little chunks of nylon in the oil pan.

Not a terribly hard job in the boat, you'll have a new timing set with all metal teeth and be able to get at the broken bolt remains.

Here's an example of a timing chain gasket set, it has all the gaskets you need, including water pump and fuel pump gaskets.

Your timing cover front seal is rotation specific so the one in an automotive set will be backwards, but you can get one separately.

Here's an example of a good timing set, your year most likely has a 2 piece fuel pump eccentric, but you can do some reading about eccentrics and if you take the fuel pump off, you can use a long screwdriver to see if you have a 1 or 2 piece ( or take things apart and verify before going to the parts store.

http://www.summitracing.com/parts/clo-c-3057x" rel="nofollow - link

Your favorite local parts store should be good for everything but the timing cover RR seal.

A harmonic balancer removal/installation tool comes in handy also, to prevent damaging the crankshaft threads.

There a lots of pretty much universal harmonic balancer pullers that will work good and in the link is one example of a Ford harmonic balancer installation tool.

With the puller, you want a good cone shaped piece on the end so you don't mess up the threads

The installation tool in the link has the right threads for your crankshaft.

http://www.jegs.com/i/Moroso/710/61742/10002/-1?gclid=Cj0KCQjwsLWDBhCmARIsAPSL3_1gEScTcy8kn3QaXsiKdrRnXgAxcxvKkAX72Deoe0XS9f77QuOKMdEaAplHEALw_wcB" rel="nofollow - link

Here's a link to the seal at SkiDim

Here's a picture of the one piece and 2 piece eccentrics. 2 on the left 1 on the right

If you want to find out before disassembling everything, take the fuel pump off and with a long screwdriver push up on the bottom of the eccentric.

If there's no play..............it's one piece

If there's some play........it's a two piece

If you want to gamble, you figure it's a 2 piece eccentric because Ford went to the 2 piece in the early 70's on the car engines. 

it was a money saving deal or a friction saving improvement depending on who you talk to.

As far as the bolts, there are a whole bunch of different lengths, so keep track of what goes where and I've had good luck with Grade 8 and don't forget the anti seize.

You'll have to check the timing and maybe reset it.

Torque values............buried in the chart in the link, you may find other values in different places, but they should be relatively close

http://www.torkspec.com/torkspecme.aspx?KI=11-5.0L-302ci-V8" rel="nofollow - link

Or click the below link  and go to page 8-07 of the Ford manual from the CCF reference section

http://www.correctcraftfan.com/Downloads/Ford_302-351_Service_Manual.pdf" rel="nofollow - link

As I will also be pulling and reinstalling my harmonic balancer shortly: https://www.amazon.com/gp/product/B079GF9H9Y/ref=ox_sc_act_title_1?smid=ATVPDKIKX0DER&psc=1" rel="nofollow - this installation tool is very reasonably priced on Amazon. Looks like it should work for a Ford 351 as well, or am I missing something?

I think you're missing nothing 

In the  description in the link below, it says it has 5/8-18 threads on one end and 3/4-16 on the other end, making it a pretty clever "multi purpose goes both ways" tool     You'd thread in the 5/8-18 end for your engine

http://www.proformparts.com/product-detail/66522/harmonic-balancer-installation-tool-fits-ford-pontiac-buick-olds-crankshafts" rel="nofollow - link

2 piece eccentric in there?

Getting broken bolts out sure can be a problem. You probably lost some threads by resorting to drilling and tapping so is there enough depth to get a longer bolt in there? 

Maybe bad surfaces where the pump back cover contacts the timing cover at those ports