I'm with Mike on the springs. After having floated a valve on a missed 2nd gear shift, breaking the valve off and totally destroying the stock engine I learned my lesson. No, not taking it easier. Bore it, cam it, heavy duty valve springs and continue to flog the #$%* out of it. The heavy duty valve springs have lasted since about 1980 and have seen MANY runs past 10K in that time without a problem. Cheap insurance in my opinion. The heavy duty springs MAKE the valves close at high rpms rather than doing what they want due to the inertia that is created. It just isn't a good thing when the valve stays open too long and kisses the piston due to a weaker stock/old valve spring.