Quality engine swapped Fieros don't sell for anything close to what it cost to do the swap. Prices have been all over the place but in most cases the V8's do cost more. Now with a kit car, as you are intending to purchase, that's a big unknown. The is no Blue Book that can be referenced. The only thing that's safe to say is that an 88 kit car should sell for more than a 84-87 kit car. Maybe Rodney Dickman who maintain the Mera registry would know.
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "