[toddman67]

I use moldeling clay to check clearences. Put it in the valve reliefs and install the heads only snug or use an old gasket. Remove the heads and measure the clay thickness with a micrometer or caliper. Don't forget to substract the gasket "crushed thickness" compared to the original gasket thickness. As with many gasket makers, there may be several different thickness gaskets to obtain the clearence that you need.
The correct method here would be to have pistons with reliefs that accept the valve size.