Now, I have another question. Will the U-joint on the drive shaft that was for his 8 1/4 rear end bolt into the yoke of the 742 rear end? Also, will the drive shaft be the correct length? Keep in mind, presently 318 with 904 transmission going into an 8 1/4.
If he wants a 8-3/4 SG, he will need to get the housing (b body width) and center section. There are c body axles that are not tapered that can be shortened and the housing can be shortened to the right size if you can't find the b body size in good shape.
I do everything myself, except setting up ring/pinions... tools exist to make it pretty mundane, but if you're only going to do it once - probably best to farm that out if you need to switch gear ratios.
The yokes come in two different sizes, one larger than the other (7290 and 7260). You can measure the distance edge to edge to figure out which one you have. You can also swap them out to match. They also have conversion joints, that mate small to large. Depends on what he is comfortable with. . The driveshaft might need to be a different length, something to check when the new rear is in the car.
If it were me, new driveshaft, good used large slip yoke, good large used or new rear end yoke, and new matching ujoints. Do it once and be done.