Rear main seal.

https://www.alexsparts.com/rear-main-seal-marine-reverse-rotation-351w-viton-full-round-one-piece-main-seal/
I think this is the right one..

If it's a Ford, the changeover from 2-piece to 1-piece seals was made about 1984.

Fel-Pro makes and sells the two piece and the one piece standard rotation seals.
On the two piece are they different for std and rev rotation?
Fel-Pro is available at almost every Auto Parts store and can be ordered even if they don't have it in stock and usually same day or next day delivery.

2 piece seals for your 351w engine don't care which way the engine rotates but some places want to sell you a "special" 2 piece reverse rotation seal for a "special" price.

If you use a standard rotation 2 piece seal that has the grooves in it you will have oil in the bilge. The reverse rotation seal is no longer available. That is why you use the smooth 2 piece seal.

Correct, Scott. (See below)

MrMcD wrote: Fel-Pro makes and sells the two piece and the one piece standard rotation seals. On the two piece are they different for std and rev rotation? Fel-Pro is available at almost every Auto Parts store and can be ordered even if they don't have it in stock and usually same day or next day delivery.

According to the literature I have from PCM (see my post) the 2-piece seals are rotation specific. As I understand it, both directions are rotation specific, but the smooth ones are not. I don't know what is available recently.

The wicker lines are know as the Helix, at least that is what the Crankshaft grinders I knew called them. They are designed to use the rotation to wick the oil back away from the seal and into the engine oil pan. Some cranks use the Helix, most to not.
Some seals have similar lines built into the seal face for the same reason and makes them one directional. If used in reverse they actually help oil pass and leak.

ArtCozier wrote: MrMcD wrote: The wicker lines are know as the Helix, at least that is what the Cranshaft grinders I knew called them. They are designed to use the rotation to wick the oil back away from the seal and into the engine oil pan. Some cranks use the Helix, most to not. Some seals have similar lines built into the seal face for the same reason and makes them one directional. If used in reverse they actually help oil pass and leak. Spot-on correct. In addition, looking at the photo in the post previous to yours, if I am looking at the photo correctly and the crankcase is to the left in the photo, it appears to me that the seal if facing the wrong way. The lip of the seal is at the very edge of the "wick lines" while if it was turned around it would be in the center of them. My experience with seals has always been that the lip faces the oil to be contained.

I agree 100% with youArt, on the seal being installed wrong in the picture I posted of mbshaw's seal installation. The lip always goes in towards the inside of the engine. The lip rides right over the wick lines, illogical as it might seem.

Kinda wonder how his seal is doing these days.

The FelPro BS 40042 has nothing in the rubber to make it directional and works with either rotation these days.

And the directions say the lip goes in unlike that picture I posted.

I'll post a picture of the perfectly smooth BS 40042 and ask that somebody post a picture of a Ford 2 piece seal with wick lines in the rubber.

2 piece Chevy seals are a good example of the lines being in the rubber

Look Inside the seal. This is a 2 piece seal, there should be an outer lip designed to keep the dirt outside and an inner point that is your actual sealing surface.
Fel-Pro would advise you to install that seal slightly rotated, 3/8 inch so the parting lines of the seal don't line up with the parting lines of your main bearings.
Use a very small film of silicone on the parting lines, where the upper and lower 1/2 of the seal touch each other. It helps seal a potential leak path. If you use too much it will create a leak or if any gets on your helix it will leak the main cap and the block should be clean and dry where they mate and a very thin film of silicone should be used there also to keep the block from leaking oil between the cap and block. so be careful with your application and work quickly once you apply the silicone so it is not starting to set up before you torque the main caps.
Of course some assembly lube should be used on the crankshaft bearings and the new rear main seal before dropping the crank in. If you don't have assembly lube good clean oil is OK. Many rear main seals are ruined on start up when they are installed with no lubrication. Rotate the crankshaft after the bearings are torqued. It should rotate free and pretty easy. I hope you get it running again soon.

Well I'm going to pick up the seal today after helping a friend get some stuff done. Fel-pro 2 piece seal.

Sorry Keno, I am a Chevy guy, 2 327's, one 350, one 400 SBC and a 5.3L but my 351W Ford has not broke so it is still assembled.
The seal in your picture looks like one of the hybrid seals, a modern version of the old rope seal, it sits in a smaller groove with no hard outer case.

I do have contacts in Fel-Pro, and they have a help line that has good techs with access to blueprints and engineering if needed.
1 800 325-8886 M-F 7:30-4:30 Central time.
I did not realize the Ford stayed with the original rope grooves for a Rope Seal.   Why not run a Rope Seal they are bi directional?   
After leaving the note earlier I was waiting for someone to point out the FORD will keep running so I do not need to know what is inside.
I really like the 351W for a Boat motor it is near perfect. My first Ford 351W 78 Nautique needed heads at 1100 hours but with new heads still ran like a top when I sold it with 1,500 hours.
Put 800 Hours on a 350 Chevy in our Malibu, it never needed anything at all in that time other than the impeller every other year. Still ran like a top when I sold it.

So kinda summing this up, the FelPro 2 piece seal BS 40042 has no wick/helix lines, never did, never will and it works fine in either direction.

It's not new news, people have used them for years in normal rotation and reverse rotation engines with no problems as long as the seal installation is done correctly.

A 351 engine with a one piece seal is a whole different story not meant for this thread but discussed plenty of times here onCCF

I ordered the feel pro 2 piece seal and it came in yesterday. Didn't have time to pick it up. Florida gives kids this week off for Thanksgiving. Never did that when the majority of us went to school. It's caused some issues in getting other things done. Anyway,ill get some pics up asap guys.