84-87 hurst and gns used 8.5 78-84 used 7.5's.I have put the 8.5 in a 79 which came out of a 87 gn. There was no difference that I can remember, other then wheels studs were metric on 87 and 79 was not. The only other thing i think was different was the brake lies,not for sure...Reason I know this is because my brother has a stone stock 78 turbo sport coupe which has blown up the 7.5 ,3 times so when we found a 87 gn local we purchased the 8.5 out of it. Hope this helps