The IROC replaced the Z28 weezl... lol. When the IROC name was dropped to go with Daytonas, the Z28 status returned. The 350 they had were not bad, but overall as a platform, Supras are much easier to build.
My 84 Z28 handled like a god damn dream, and for auto'xing, id take it over a supra any day of the year. But if you want just pure speed, without having to upgrade the rest of the car, the MkIII is so much easier.
Doward, btw, do you realize how damn hard it is to swap in the t56 ls1 combo? its a nightmare lol. Easier to just build a crazy fast 350 and then do a t56