Assuming that the oiling system was working, you had good oil pressure and you could see a good amount of oil on the rockers, valve springs, etc, most of the engines that I�ve seen wipe out the lifters as you described, was caused by valve spring bind. Valve spring bind is caused by a valve spring setup that can not handle the lift of the cam and rocker ratio and the springs bottom out before the cam can fully open the valve.
Mike
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