My 'Bird has a 428 engine. This winter I'm rebuilding the engine vs. replacing because I enjoy the idea this may be one of those Factory/Dealer conversions. I realize the liklihood is rare, but what the hey? I bought the car from Daniel Schmidt Dealership in St. Louis in 2005. The Build sheet from PHS identifies Zone 12, Dealer 596. After much research, I was able to determine this Dby 68Firebird428 - Pontiac GTO Forum