If you are going to raise your compression ratio, you should check your piston height above the block deck so that you establish a proper, uniform quench band (some people call them squish bands) in the combustion chamber. This will go a long way to reducing / eliminating the detonation problems that may come with increased compression ratios.

I seem to recall that the correct piston height at TDC is 1 - 2 thou above the deck. I am sure that others who have built racing engines will be able to advise on the correct deck height. In order to achieve a uniform height, you may have to mill the cylinder deck so that the lowest piston is the correct height and then mill the tops of all the other pistons so that they are a uniform height. It is not a trivial process.

In order to determine your compression ratios, you need to cc your combustion chambers to determine their volume. Hopefully they are reasonably uniform, otherwise you may have to do some grinding / filling to get uniform clearance volumes. With the combustion chamber volume, compressed gasket height and height of the piston you can calculate the clearance volume. With the swept volume this allows you to calculate your actual compression ratios. You can then calculate whether you can reach your target CR by changing head gasket thickness or whether you will also need to mill the head.

Be careful with milling the head. Milling changes the combustion chamber volume; but, because of the irregular shape of the combustion chamber it can be tricky predicting how much a pass of the milling machine will alter the volume. To be safe, you need to mill, cc the head again and then recalculate the change to the CR. Repeat if you didn't make your target. Also, now that you have established the correct deck height, you have to be sure that you don't mill too much and mess up your quench band.

Take measurements and plan everything out ahead.

Something to consider, if the engine was previously balanced milling the tops of the high pistons will alter engine balance. If you are lucky, the pistons will be uniformly low in the bores and all you have to do is mill the block to get a reasonable (perhaps not perfectly uniform) deck height and leave the individual pistons alone.