Will the transmission on a 3.8 99 Pontiac firebird bolt up to 350 Chevy?
No, a Pontiac 3.8L V6 transmission will not bolt up to a Chevrolet 350 small-block V8. They have completely different bolt patterns on the bellhousing. You'd need an entirely different transmission, and likely a new flywheel and possibly other components to make the swap work.