he's got a 22R with a 20R head on it harry <img src="/forums/images/graemlins/smile.gif" alt="" />

i'll go over some options:

5M/6M/7M variants:
pros - bolts to 22R engine mounts and W/G series trannies with the stock bellhousing, keeps it all toyota, lots of power and flat, smooth powerband
cons - inline 6 engine, so it's long. exhaust is on the wrong side and must be re-routed. but it's a reasonably popular swap

chevy 4.3:
pros - cheap, easy to find, no shortage of power, lots of SBC parts interchange
cons - much heavier than 22R or buick motor, distributor in rear, longer/wider than buick

buick 3.8(231):
pros - reasonably light, front mounted distributor, decent aftermarket support(though its kind of fallen by the wayside since everybody wants the bigger 4.3 now)
cons - i've heard they run hot, have some head gasket issues. the newest RWD block(unless you buy a spiffy $3k aluminum TA race block) is about 15 years old. expensive to use a manual tranny unless you can score something like an old monza bellhousing and flywheel

ford 302:
pros - 100lbs lighter than SBC, narrower and shorter than SBC, front mounted distributor, won't necessarily chew up your drivetrain right off the bat
cons - it's still a v8 so it's a tight fit, can't use the strong R series tranny so you'd need a T18, NP435 or a slushbox