The standard shaft length is 50 inches which is probably the 50 stamped on your shaft.

They also had lots of distance between the strut and the prop hub. It could vary depending on the prop you have on when you're measuring it but something in the 2 1/2 inch range plus or minus a little, sounds right for a 76

If you don't have any vibration issues with the worn strut bearing, taking into account the "fun" involved in removing the coupling and the "double fun" involved with reinstalling it with the engine in the boat, I'd run it just the way it is till you get a double tapered shaft. Then you cut the old shaft right behind the coupler and remove it from under the boat and install the new double taper  shaft You'll have to remove the rudder or remove the strut to allow sliding the old shaft out and the new one in since the rudder is straight behind the shaft

Or............You could remove the strut from the boat and slide it off the shaft and replace the bushing on the workbench, then reinstall the strut and check your alignment. You'll probably have to unbolt the coupling from the transmission to give enough wiggle room to slide the strut off the shaft and back on or you can remove the rudder instead.

Another choice that sounds easy but is kinda hard is buying or building a special tool that clamps around the shaft and can push the old bushing out and push a new one on with the shaft installed

You can shorten the bushing to fit the strut with your favorite cutting tool.

A general thumbrule says you want about 10% of the prop diameter for your blade distance from the hull.

That would be about 1.3 inches in your case, you're a little closer but that's OK.

You've probably read on here that the prop hub should be about 1/2 inch from the back of the strut with fancy terms like "minimizing overhung load" being tossed around

Now if you try to go shorter with the shaft, that distance between the blades and the hull gets closer and if you try shortening it any from your 50 inches, you'll run into hull clearance issues

That's just the way CC did some shafts back then.

It been good for 40 something years the way it was built with all that distance between the strut and the prop. Not ideal, but hard for you to change with the measurements you've got

PS good luck with that tool bar request 

It was a 78 Martinique getting a new strut bearing among other things.

On page 2 post 2 you'll find reference to the same strut number AF 25006.

I'd figure it's original and stamped on at least some 154 DV struts over the years
 

For some strange reason I keep an eye out for black Mallory springs and an old CC gas cap 

All this easier with the strut off the boat. 

Most people install the factory type rubber new bushing.   I tried the new Delryn plastic bushing available.  Delryn is used to make hip joints and even used in ball joints on cars so it is very tough stuff.   I only have 100 hours on it so far but it seems to have worked very well.   Guys run the factory type over 1,000 hours so I have no idea if this plastic bushing will last that long.  It does offer near zero drag so that is good.  If you choose to use the Delryn type bushing be aware the set screws need to hold it in place but the plastic is flexible so when you tighten the set screws it collapses the plastic.  I ended up drilling holes in the plastic where the set screw needed to seat and tapping the plastic hole to match the set screw.  On install I put red locktite on the threads so it would not work loose.   Only 100 hours of use but good so far.

Holeeey  ****   congrats

You got a picture posted. 

Might need an explanation of how

I won't ask if the boat is upside down