7 3/4 to 8 3/4 swap only

I did the swap in a '70 Dart when I was 17. I was disappointed to find out that with everything bolted up, the driveshaft would not fit. It was not off far so I ended up taking the rear back out & drilling new holes in the perches for the alignment dowels in the leaf springs to make it work.

Your 7 1/4 rear should have the small u-joint, but 8 3/4s can be small or large. You can get a 'crossover' u-joint from big to little if needed.