You will need to get the trans and axles for the supercharged motor this i know for sure more than likely the computer and possibly the engine harness also. Don't quote me on this remember its labor day so a lot of the members are not around or are unwinding from their weekend. Just be patient and i'm sure you will have all your questions answered soon.
Or another route you could go is a top swap judging by your saying the oil pressure drops i would replace the oil pump.150k miles is nothing for a 3800 I have a 94 Bonneville se with 453 thousand miles.
2000 Suburban 3" lift and some power mods..The daily
06 impala SS CAI,Exhaust,pirellies the other daily
86 camaro Far from stock......The hot rod
1992 camaro RS 25th anv 350 5 spd..The cruiser