49-53 minimum cast iron head thickness??

[Init1]

Hey flathead experts...

I have run into a problem with my fresh 50 Merc. 2 exhaust valves just very slightly kissed the head . No damage but what is the minimum thickness for cast iron heads and where and how do you measure it?
My heads have been milled flat at least once and probably more over the last 65 years.
Also who makes the thickest head gaskets. I have been using BEST
graphite.

Thanks for the help...