[Sirrotica]

Before you go trying to put a blower on a 66 engine, be forewarned that they have thin wall cylinders on 65, and 66 389s. That block is probably the worst choice to stick a blower on of all Pontiac blocks.

If you want the largest combustion chamber the 71-76 455 smog heads are the largest combustion chambers. The valve angle changed in 1967 so the only way you could use the 455 heads on a 389 is keep the valve lift under .450 so the valve doesn't kiss the piston due to different angle valve reliefs. The valves are larger valves on the 67 and later heads. The 389 notches aren't in the right place, they're the wrong inclination angle, and they're too small for the later heads.

Any of the 67 and later 400s have much stouter cylinder walls, as do 428s and 455s. These blocks are better choices for a blown engine. Properly prepped 2 bolt blocks are good for 600 HP, 4 bolt mains, or an aftermarket block should be considered over that point.