The gasser racing guys say this:
"So, how much clearance is enough? A safe rule of thumb is .080-inch for intake valves and .100-inch for exhaust, which requires a little more clearance because of the increased heat factor. Experts also suggest adding .030 to each valve if the engine is equipped with aluminum rods as they tend to stretch more than steel."
That advice may be wildly conservative as hot rodded gassers can run at 3 times the rpms that your old Lehman does. It is high rpm that stretches the piston rod and decreases clearance as well as high exhaust temps that expands valve stems. They do have a much lighter piston though, so who knows -
Go to http://www.enginelabs.com/engine-tec...lve-clearance/
and read about it yourself.