Dave, I think that is too tight. For sure on the exhaust, as any failure of the valve to follow the cam and ya got trouble. With only 0.040 piston to head, when a lot of that is eaten up at speed you are down to about .020 or .030 from crashing the exhaust valves. Any inconsistency in valve depth in the heads will add to the problem, along with any other items that are a bit off.