If the junkyard doesn't object that might be a good approach but cutting through a hardened steel axle may not be all that easy. Then you would probably have to swing whats left of those axles out of the way to drop the transmission. The exhaust crossover may also need to be removed.
Your idea is to put an automatic transmission designed for application on a 3800 engine on the Fiero Duke engine. I would check gear ratios before moving forward.
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" 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.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "