id go for a 400CI small block or a 383 stroker, both should provide the grunt to really make that monte move, and like fabertooth said the t56 is one of the best choices, with a tall rear end gas shouldnt be that bad but in 1-4 torque will still be kinda crazy, just my .02 tho man

When I think first gen Monte, I think 454, but MY64GOAT is right about small blocks.  They're probably the cheapest engine to fix up so you get allot of bang for your buck.  I got a T56 6-speed in my Chevy.  The last 2 gears are overdrive and I think the final gear is 0.5 to 1.  I can cruise down the highway doing 70 taching 700 rpm.  You get to rip through the gears pretty quick AND get decent mpg.  They're not the cheapest new or rebuilt, but they were in the later f-bodies and Vettes and some Vipers.  The clutch is a weak spot though.

my dad says go to peively swap meet. in missouri out by herculaneum or however you spell it its decent and got good stuffs

SBC 350 is always a good thing to have. They're all over so they're easy to get parts for, some what cheap to modify and easy enough to work on. As for trannies, I've always been partial to muncie 4-speeds. The only thing with them is inhaling gas on the highway if your rear gears are too tall. I got a rear about a year ago from a 96 impala that I am planning on installing in my 69 gto. It's got the factory 3.08 gears w/ lsd in it and it bolts up to A-body cars. 2 things good about that for me, 1) I get some more gas friendly gears w/ increased top speed (not really a performance problem cuz the engine I'm builing will still give enough grunt) and 2) it comes with factory disc brakes that can be upgraded later. I paid $100 bucks for it at a swap meet. Try getting an A-body rear disc conversion for that cheap. Not saying this is what you're looking for, but it's a fairly inexpensive formula for an A-body.

This may sound ove rthe top or too expensive but going with a procharged big block chevy is pretty nice.  you can take off the pully for the s/c on normal days and put it back on at the track for some good times.