LOL whatever you do, at least seriously consider ANY Pontiac engine from a 326 all the way up to the 455. Any of these older Pontiac engines will fit right in the car with no problem. If you go with the SBC you will have some modifying/changes to make etc.

The older Pontiac engines are out there, my friend Mike has bought and sold 3 just this past week alone. All in running condition and all of them complete. He sold a complete running 455 from a 73 Pontiac for $400, sold a running Pontiac 350 H.O. engine for $500.

So if you look, you will find one. This way you wont have to put up with all the hardcore Pontiac guys busting your stones over having a Chevy engine in your Bird. Not too mention ANY Pontiac engine is better than the best Chevy engine...