All I can say is, "wow, am I learning a lot from this build!". I just mocked up my 906 heads onto the 383 block, with my Summit K6400 cam and lifters, and Comp Cams 911-16 springs installed. As far as I know, the only thing non-stock about this motor are the cam, lifters, & springs. With a 0.020" nominal head gasket, and a spring installed height of 1.850", the lifter preload is a whopping 0.130". My understanding is that this preload needs to be 0.040" to 0.060" for proper operation. I used a dial indicator to measure the compression applied to the pushrod & lifter, and am sure the lifter was on the base circle of the cam. With such a mild cam, is this a surprise? I suppose now I have to get some shorter-than stock pushrods?