7 1/4 to 8 3/4 rear end swap

Not all 8-3/4 rears had bigger U-joints. Mine don't. I haven't seen many but from what I've read all A-body rears have the small U-joints. Also have a good look at the rubber brake line leading from the body to the rear. If it's original chances are it's cracked and worn pretty bad. Now would be a good time to replace it.

With your 318 the 8-1/4 would do you just fine and be alot cheaper than a 8-3/4. The 8-3/4 rears are bringing a premium these days.