IMO, to quiet down a 3800SC you would have to custom make an around the engine exhaust system with a CAT and a standard type OEM muffler. There is little that you can do to make the short out the back mini exhaust system quiet but putting an exhaust cone in the pipe to the muffler inlet should lower the tone slightly and perhaps make it more tolerable. .
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Powerlog manifold, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "