Rear end swap

The '80 Aspen should have an 8.25" rear axle, so if you have a 7.25" it would be a nice upgrade. It will work, but you'll need to relocate the spring perches. The axle is also about 2" wider than the A-body rear. Not a big deal, but you'll need about an extra 1" of backspace compared to a car with an A-body rear axle. So, typically in the back a 15x8 with 4.5"-4.75" of backspacing works, your car would need a 15x8 with ~5.5" to 5.75" of backspace or so. Those are just rough numbers though, I would definitely check that before I bought new wheels.

As far as welding hubs and re-drilling, are you talking about the front?

If you are, I wouldn't bother. If you have access to an '80 Aspen, just pull the spindles and brakes off of it and convert to BBP disks. The Aspen will have everything you need except for the large ball joint upper control arms to go with the spindles. And you can always just buy ball joint sleeves from Dr. Diff if you don't want to swap out your UCA's. http://www.doctordiff.com/a-body-upper-ball-joint-tapered-adapter.html