1978 Aspen Super Coupe, what's underneath them?

Hey guys:

I got a dude offering to trade me a '78 Aspen Super Coupe roller for what's left of my old '68 Fury convertible. The Aspen is ratty and doesn't T-Tops, so it's not of interest to me(I know there were only 531 made).

What I AM interested in is the rearend and suspension, because my civilian Gran Fury sorely needs the T-bars and 3.21 rear. What's actually under those, a 7-1/4 or 8-1/4? Steering box any better?

My G.F. is a nice clean car but it has the must underwhelming suspension and steering of any Mopar I've driven and that includes a few trucks and several B-vans.

Thanks!