The factory rated the compression ratio at 10.5:1 in 1966 with 72 CC heads. When you determine what the static compression ratio is, you will not be able to use the current cool aid quality pump gas. All Pontiac 389 / 400 / 421 / 428 CI engines and and head combinations produced before 1970 cannot use pump gas without retarding the initial timing advance to 10 degrees. You can experiment with the initial timing advance but you will find that the engine will likely have less power and run at a higher temperature with retarded timing. Try it and see. You must use dished pistons or 1971-1979 heads to use 93 octane pump gas. Otherwise use a 50 - 50 mix of 108 octane racing gasoline and 93 octane pump gas. You can successfully use E85 pump gas with up to 10.5:1 compression ratio but station that sell that stuff are difficult to find. At least they are in the Dallas area. I live in a suburb of Dallas and only one station here sells E85. My highly modified 1991 Chevy S10 Blazer 4.3L V6 has 10.20:1 SCR and 8.50:1 DCR. It has a ICM and a ESC knock sensor. I can use Mobil 93 octane pump gas with Techron to keep the electronic fuel injection clean.
Headers for a 1967-1969 Firebird will fit a large Pontiac chassis with only minor modifications.