I have my 79 t bird with 69 mercury 429 and rebuilt c6, I have to custom weld motor mounts so it would fit, so needless to say old shaft was too long.
I have a drive shaft from a 73 mustang that is shorter, the rear yoke is smaller and I only have about 2 inches of the slip yoke in the rear of the tranny.

I've been getting around by just centering the joint in the yoke by hand, and then hoping for a good ride.
the caps are the same I think, but the 73 shaft uses a 2 3/4 ujoint, and the 79 rear end, (I think its a 9") uses a 3 3/4 joint.

Can I get a odd U joint with those dimensions? sould I get a bigger yoke welded on the drive shaft? or should I have the original shortened?

I don't have tons of cash, my original shaft is at my uncles shop but thats 5 hours away from here, but i'll be going back that way in a couple of weeks,