I’ve swapped a “G” body (1978-1988 Cutlass) rear into my Jetfire to gain the 5 lug wheels and daily driver reliability. I cut the lower control arm/shock brackets off of the original, welded them in place and used 1988 GMC 1/2 ton (air)shocks. I fabricated new, adjustable upper control arms due to a slight angle change and retained the original lower arms. From what I read somewhere, the driveshaft/pinion was set pretty low on the original rears to clear the minimal tunnel area on these cars which were supposedly built on the Corvair platform. Consequently, the air shocks come in handy to keep the driveshaft from hitting the tunnel in the rear. When I put the mustang II suspension up front, I used the original Jetfire front springs in the rear. Being that I went with the newer 2004r overdrive trans, I modified a 1980’s driveshaft to hook everything up and it works well as long as I keep some air in those shocks. I also slightly dropped the trans mounting which also helped the driveshaft clearance but it still is an issue. I wanted my car to look like a 442 would’ve looked like in ’63 so I used the 15 x 7″ factory rally wheels which are too wide with the “g” rear so you’d either have to find wheels with a different offset or modify the wheel well lip. I agree that an 8.5 rear (GN) would be plenty of beef for such a light car.