DONT DO NORTHSTAR. they are not designed to be abused...lol i know from experience. I had a 95 STS,(best e/t 13.82) and after shattered shifter solenoids, rotting out freeze-out plugs, electrical nightmares, oil consumption, and corroded cooling system, i had enough.
IMO...i agree with the 3800 series supercharged swap. should fit very easily and those motors haul ass and are easily modded.
BUT if you have the cash and wanna get really crazy.,...
find a northstar trans(or similar) and a grand nat/t-type/tta 3.8l turbo engine and drop that in.... now THAT would be cool.