That's entirely up to you as to whether fear is going to get the best of you
Modern hi-po engine recommendations include approximatly 0.050" clearance between the top of the piston and the valve during normal testing and operation, as you described. What is assumed under these circumstances is that valve float will not cause the two to contact if the engine is operated at too high an rpm.
What you need to consider on your flathead is, as you stated, the thickness of the torqued gasket, but also how much you expect the valvetrain to lengthen/expand as the engine comes up to operating temperature. It can be calculated, but I can tell you that it isn't much, probably no more than about 0.001" to 0.002" if even that. And as long as the springs are good quality (preferrably new) and have about 110-120 lbs (or more) seat pressure closed, then a clearance of 0.050" is fine.
Use a micrometer on the clay to find out exactly how thick it is. If you have the old head gasket, you can put it back on and recheck by the same procedure. If you still don't feel comfortable, you can have flat valve reliefs cut into the chamber that smooth out the chamber dome over the valve.
Hope this helps, and give a holler if you have a question.
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David
newbie #4153
