I have an opportunity buy a complete drive line out of a 86 Monte SS. Would I get any better performance if installed in 87 Elky with overdrive.

Depends on what your running now. What you'll get out of the Monte is a 190(I believe) horse 305, a 2004R overdrive transmission and a 3.73 geared 7.5" 10 bolt rear end probably with out a posi. If it were me I'd take the tranny and rear end and find a 350 to put in front of it.