Well, I may be able to pick up a B-Body 8 3/4 rear for cheap. ($75) - Certainly less than I can pick up an 8 1/4 to bolt in.

The 7 1/4 in the Volare we just picked up for my 15 year old is making noise and has some metal in the oil. If I'm going to change out, what must I do to put the B-Body. 1/2" narrower on perch, 1/2" wider at the drum.

Will they just bolt right in? Any custom mods needed?

(link from AspenVolare forum)

I did this swap 20 yrs ago. I put a 70 roadrunner 8 3/4 in a 76 Volare. If I remember correctly I had to toss the rubber mounts from the car and use a replacement spring locater stud from the roadrunner. I used a adjustable pinion snubber. I believe the driveshaft was too long so I had the roadrunner shaft shortened. I had a 340 4 spd in the car and it was a real sleeper. Overall it was a pretty easy swap and worked well. Gerry.

I have a 68 B body 8 3/4 in my Mirada which has the same setup as an F body. It's pretty much a bolt in but you have to change a few things.

Right now you have rubber isolator clamps between your axle and springs...This setup uses large center bolts so the holes in the spring perches are over 1". The 8 3/4 perch holes are only 1/2" so you'll have to drill them out bigger to fit.

Another option which is what I did, is to remove the clamps all together and use the older 8 3/4 shock plates with regular 1/2" center bolts and use B body shocks.
B body spring perch width is 44". The F body is 44.46, which is less than 1/4" per side difference. The springs easily move out enough to drop the 8 3/4 on no problem.
Keep in mind your driveshaft will now be too long as well.