Yes Grim, admittedly I was running a street forged pistons which had greater clearances due to a greater piston expansion rate, And I was running 6400 RPM with hyd camshaft which all adds to the pistons hitting heads and piston rock. What I am saying is, I would leave this to the drag racers and stick with more piston to head clearance for a reliable street motor. 
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It all depends on what you intend. I’m making a rather mild 331 stroker that isn’t likely to see the high side of 5500 RPMs, but I want it to run on cheap gas and last forever.
I sure agree that at 6500 or 7k, I’d be a bit nervous at .039! You need more clearance to account for crank flex, rod stretch, all that.
I am right at .040 on mine, with 9.3:1 compression, running 58cc heads. (Zero decked, .040 cometic head gaskets, Mahle forged pistons)
Guess we’ll see how that goes. =)
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