[Mike C]

That casting number is for an open chamber tall deck 427 head from 68-76. If an original 68 396, that head may be killing your compression ratio. I did not find out what cc the head was, but the closed chamber heads would have been on the 68 motor.

My 69 L36 390/427 has 100cc chamber heads for 11:1 compression. If the factory 396 heads were 100cc with 9:1 compression then the 401 casting head would make for 8:1 compression if it is 110cc.

You need to do more research and figure out exactly what mashup of parts you have.