It would be fun to experience how an original 400 HO 4-speed 1968 Firebird performs.
If you strive to build it as factory correct as is practical then you’ll have a nice fast car that’s reliable as well. Date codes and such aren’t important really.
Sometimes it can be rewarding to resurrect a classic high performance car and see what it would have been like to drive one exactly as delivered back in the day.
I did exactly that with a 421 HO engine. Even though my ‘64 Le Mans convertible was never factory equipped with a 421 HO I wanted to experience the feel of one that’s unmodified. Other than adjusting the compression ratio to work in harmony with today’s fuels it’s just as the factory would have built it.
Stock can be satisfying, especially with a Pontiac High Output engine.
A 455 is a nice powerful engine but the 400 HO is better suited for a manual transmission in a lightweight Firebird. The 400 won’t blow the tires off as easy and rev a little higher, a recipe for fun.