I have to agree with the above, it is your car, do what you want. That said I also go out of my way to have a Pontiac in a Pontiac, a Buick in a Buick, etc. I would locate a Pontiac V-8, and keep the Muncie and rear end. Unless the rear is some crazy gear ratio. Chevys need lots of gear to make power. Pontiacs make power down low and can push highway gears. I feel a 1968 Firebird convertible deserves nothing less than a pure Pontiac engine. PS, if it really got the 80's Chevy treatment, it is probably loud and obnoxious. Are you happy with that?