pretty much all the chevy 8'* had the same bolt pattern. i dont know about the six. i would start with the engine because what you go with there will kind of determine what you want to do with the trans. like he said you could go with a whole setup from something modern, i think if i was going to do that it would be a ls engine with the t56 six speed you can get them as whole peices or find a wrecked one, you would need a whole bunch more than just the engine and trans.
or you could go with a old sbc, you can get a crate engine for pretty cheap and a muncie 4spd. not a whole lot of wiring and that sort of thing to get it going.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock