It sure seems like the lifters are "pumping up" and causing the valves not to close all the way. I never have seen that happen but I've heard of it. Did you check for broken valve springs? I know when you overheat aluminum heads they can warp bad and if there has been head gasket work on that truck and had the head milled more than once, the rocker arm may be sitting too low and the lifters are near their max compression.