7 1/4 to 83/4 swap - A Body
When removing a 7 1/4, and installing a 8 3/4, the driveshaft needs to be shortened.
There are two different yokes possible on the 8 3/4 rear. I forget the number designations, but there is a large and a small. On an "A" 8 3/4, it should be the small yoke, unless it had been changed in a prior life, that is the same as on the 7 1/4 rear. If the 8 3/4 rear has the large yoke, there is an adapter u-joint available, at most part stores. Don't remember the part number, but if you need one, send me a pm, and I will look it up.
I just installed one of these joints on my 66 Valiant, when I installed a borrowed 4.88 chunk that had the large yoke.