This website has a wealth of info. Scroll about halfway down, and you'll see Body Tag FAQ's.
http://firstgenfirebird.org/firebird/FAQ/
If the owner claims it was a 6 banger converted to a 400, be cautious. Once the PHS documentation comes back, and it's shows 6-cylinder, it will be very difficult to ever resell. If it's a driver, that's great, but if you plan on buying it, pay the clone price, not the authentic price.