You need to measure your head volume before anything is taken off it.
I did three heads awhile ago and got three different results.
All heads I assumed were stock but even the factory changed compression over the years so it’s hard to know what is standard.
Also, because an engine might be a latter model don`t assume it will have higher compression. The nos engine I have has a 6.2 comp and I believe that is a mid 60`s flat head.
results I have,
Year, Head thickness, head volume.
53, 1.925", 6.065 cu"
58, 1.885", 5.639 cu"
nos, 1.966", 6.896 cu"
End result if I use the nos block which has 0.010" of the block (standard) with the head from the 58 (looks to be about 0.080" off from the nos head) I will end up with about 8.124:1 compression.
Another area that is sometimes forgotten in doing these calculations is how far down the bore the pistons sit. On the 251`s I have done its 0.017" is normal on a 50`s standard block.