It looks like you are doing a terrific job of mating the SBC to the TH125 three speed. It should work out well but be advised that without some internal modifications, that automatic trans is not real strong. The good news is that they can be built stronger by adding a HD clutch pack, and shift kit. We built one a while back and it held strong at WOT with 250 hp engines. The other option is the 4T60 transmission that is stronger. Good luck on the project.
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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 "