Those were 670 heads with the casting number on the center exhaust ports. They were performance heads that came on 1967 GTO, 428 H-O, and Firebird 400 engines. They are excellent heads and are sought after by the racing fuel crowd. They have 72 CC closed combustion chambers which will produce 10.20 - 10.5:1 compression ratio ( about 8.5:1 DCR) on a 400 CI engine. That CR and DCR is borderline for today's 93 octane pump gas. You may get by with that CR on a fresh engine but as the mileage increases, detonation will become a problem if the initial timing advance is set at 12 deg. BTDC where the engine will run cooler and better. You will need dished pistons for a daily driver on pump gas therefore you would be better off installing 1971 - 1979 heads from a 350-400 CI engine and keep the pistons you already have.
Last edited by MouseFink; 11-16-2012 at 05:14 AM.