The question of head differences comes up every once in a while and most want to know if they can put the 1967 and up 400CI heads on a 1964-1966 GTO 389CI or big car 421CI (the family of engines with oiling through the pushrods and not the block).
I ran across these diagrams/pics that show the differences between the 1966 and earlier heads and the 1967 and up later heads. The 1967 400CI heads were closed chamber except for the mid-year big car "061" heads which were open chamber heads. 1968 and up were all open chamber heads.
The pics show the angle of the valves and their orientation in the head and above the piston - 20 degrees versus 14 degrees. The differences in valve angle, size, and placement clearly point out that the piston's valve reliefs need to match that of the head used.
The larger 2.11" intakes and 1.77" exhaust valves on the 1967 and up high performance heads required more room, so the valves are moved closer to center of the cylinder and you can see the difference in the centerline distance between the valves in the last pic.
The 1967 and up heads use a pushrod guide plate seen in one of the pics while the 1966 and earlier heads used the pushrod hole in the head itself to keep the pushrods in place. And of course screw-in studs versus pressed-in studs when going to the '67 and up high performance heads.
To use the later 1967 and up heads on the 1966 and earlier engines, options are to obtain a set of pistons with either the correct valve relief positions or select a piston designed with valve reliefs that allows the use of either type head. Another piston that may work would be a dished piston used to lower compression. In each case, valve-to-piston clearances should be measured to ensure that there is enough clearance so valves/pistons don't contact - especially with a higher lift and/or longer duration camshaft.
Cylinder bore is another consideration. The large valve 1967 and up heads require a larger bore so the valves do not hit the cylinder wall nor become shrouded by the cylinder wall. The 389CI can be bored .060" over to 400CI bore size or the cylinder can be notched for valve clearance. The 421CI would require a .030" over bore or notched cylinders.
I read that if the lift is not too high, you may get away with installing the later 1967 and up heads on the earlier engines, but I would definitely be checking valve-to-head & valve-to-cylinder clearances on that one.
Here is an older post that details a magazine article in the 389CI to 400CI conversion: https://www.gtoforum.com/f12/389-400-conversion-118561/