birdman,
Welcome aboard! Too bad the car was misrepresented, but you got a keeper there! Firebird is the cream of the GM pony-car crop.
No, you can't bore a 350 Pontiac to make it a 400. The external dimensions of the blocks are the same, it's true. But the castings are not identical. Wall thickness is as much, if not more an issue with them, as with that era of small block Chevy.
For the engine, I would recommend first things first. Arm yourself with current and accurate information. Jim Hand's "How to Build Max-performance Pontiac V8s", published by SA Designs, is the best book out there today, for a street Pontiac. Lots of good information there!
Performance Years and Ames Performance are also your best sources for restoration parts. Their prices are competitive and they don't sell Chevy parts. Pontiacs ONLY. Many of the resto-houses sell both, and you get what ever they think "fits". Camaros and Firebirds are similar, but not identical.
I happen to be a Pontiac "bigot", so feel free to PM me or post any questions here. I'll keep an eye on ya!
Jim