An E-body 8 3/4 is too wide if you want to use any kind of 14 or 15" rims that don't have a custom backspace. I have a 68-70 B-body 8 3/4 in my Duster, it works out to be 60- 1/8" wide wheel mounting surface to wheel mounting surface. I ran 15x7's with 4.25" of backspace on it with 225/60/15's and even those were right on the quarters. There's no way I could have gotten 235's on it without rubbing, and the E-body rear is an extra 1.5" wider compared to the 68-70 B body 8 3/4, so, that's an extra 3/4" per side toward the quarters. Not even the 225's would fit with a standard rim, you'd need to have something custom made and even then you might not be able to get as much backspace as you'd need because of the actual design of the rims.
No sir! You can NOT interchange SBP and BBP brake parts, doesn't matter if they're both 8 3/4's.
The brakes depend on whether or not the 7.25" is BBP or SBP. If it's BBP then you can use that drum brake set up on the BBP 8 3/4's. If it's SBP then you can't use anything with a BBP 8 3/4, and you'd have to source a set of BBP brakes. You can not interchange the 5x4" SBP brake stuff with the 5x4.5" BBP brake stuff unless you also change out axles, the axle flange offset is different for the different bolt patterns and so is the offset on the backing plates, drums, etc.
For a Duster, a small block/904/8.75 car should use a 49.25" long driveshaft. Should being they key word. Ride height, what springs you're running, how everything fits on your car can change that. I would always recommend measuring your own car before you buy a driveshaft or have one cut down. But that was the factory length.
