Your older head will swap over to your 77 motor just fine. However, I don't think you are going to get much of an increase in compression with the older head. Slants are notorious for low compression from the factory. The only way to verify this is to CC both heads, see how for down the hole your piston is at TDC, and do the math for each head. I suspect you will need to shave the head to accomplish much in the compression arena.
