By casting and date codes on the block and heads. You compare the casting codes to what your car should have and the date codes to the build date on your firewall tag.
You can post that info here and we'll help you out if you wish.
This sight here is a little messed up, but it shows photos of the places you should be looking:
And we might want to start by verifying it's actually a factory 400 car first. We can help there, too - but getting the documentation from Pontiac Historical Services is a good idea.
Welcome, by the way.
2012 Mustang Boss 302 #1918, Competition Orange. FGF replacement 2006 Mustang V6 Pony, Vista Blue. Factory ordered. 2019 BMW X3 (Titled to the wife, but I'm always driving it for her. So I'm claiming it) Old projects, gone but not forgotten: 1967 FB 400, original CA car. After 22 years of work, trashed by the guy who was supposed to paint it. I had to sell it. 1980 Turbo Trans Am 1970 Mustang fastback, 351C 4Bbl, auto 1988 Mustang GT, 5 speed 1983 F-150 4x4, built 302 1994 Chevy K2500 HD 4x4, 454 TBI
