Best bet is to get on the HP Tuners forum and ask around. Adding boost to a 3800 and tuning for it is not a difficult task but few do it right. If you have the HP Tuning Suite you have the means to scan take readings and adjust the PCM accordingly . While the DIY technique may not be what you are looking for, there is loads of info on the HP Tuners forum that could be put to good use.
" 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, 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 "