Just mill the intake side of the head an equal amount and you should be golden.
The 'vintage' (40-years-old) advice from HO Racing and Nunzi, back in the day, was to mill both the head/block surface AND the head/intake surface equal amounts.
If I recall their description correctly (without digging out the book), because the Pontiac block is a 90* block, simply removing the same amount on both surfaces keeps the distance between both intake mounting surfaces the same in relation to one another.
I've honestly only ever done this once, on a pair of 1971 455 big car heads (I think they were '66's), many years ago. We took off .060" on both sides and everything fit just fine.
Good luck!
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