If you are not sure about putting a Pontiac 400 into a Firebird, here is a web site that can give you some information that may help you with this question: http://www.thirdgen.org/techboard/engine-swap/291473-pontiac-v8-swaps.html.  This next site talks about all the Firebirds, and if you scroll all the way down the page you will find information about the third generation that might help you answer your question about  what will fit under the hood. http://www.edmunds.com/insideline/do/Features/articleId=68211. Finally on this last site you will see that it talks about Firebird First Aid.  If you click on the link that says Technical Articles, there is all kinds of information about which engines to swap and so much more about how to repair the sagging doors, what to do about the coolant systems, and other issues that concern Firebirds.  Everything you can ask about a Firebird, is on this site.  If you don’t get the answers you are looking for, just contact them, through the site, and you should be satisfied.