If the heads have not been milled since new, they will likely be about 77 cc. You will end up with compression ratio above 10:1, but probably not the advertised 10.75:1 for a '64 GTO. In any case, this is much too high for a cast iron Pontiac head--closed chamber--for today's pump gas.
Until you measure the chamber size after polishing and valve seat grinding, you can't be sure of what actual compression will be. Anything over 9.5:1 with those heads will be a problem with any kind of pump gas now available.
The only answer I know of is dishing the pistons an amount needed to give about 9.25:1 comression ratio--unless you don't care about originality and switch to aluminum heads which will tolerate a higher compression ratio without detonation.
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