Technically,(as far as I know) there is no big block/small block Pontiac engine. The same basic block/head casting is used for the 301/350/400/455.

As far as the most common swap around here anyway, is to use a Pontiac 400 (455 if you can find one). Here any way, you can pick up decent running 400s from full sized Pontiacs for next to nothing, it's a bolt in swap and the car definatly retains a much higher resale value.

Parts for the Chevy swap should be easy to come by. The Camero of the same era are the same platform, and the motor mounts, exhaust, oil pan etc will all bolt in.

Streets is right about the tranny, and you will need to pick up a Chevy tranny unless you have the Multi-Bolt pattern one in the car.

I have a dailey driver 81 Firebird (TransAm clone) that started life as a 4.9 car. It has a stock 400 4BBl from a 78 Catilina in it with stock manifolds. It runs an regular gas, get about 21 MPG on a good day and has never lost a race to even mildly modified Cameros, plus has a higher resale vale.