Passenger side rear ----------thats where the distributor sets in its boss/hole-------IFFFFFFFFFFFFF------the dist is not exactly straight into hole( manifold at top not centered perfectly in line) lower housing undersized, oil rings ( if used) bulged out into oil galley, and/or a few other conditions-----------at low speeds the flow of oil to those rear tappet bosses can bleed off enough that there isn't flow/pressure enough to do the job of filling the tappets at a rate that beats the collapse rate of the lifter-at higher speeds the lifter isn't in that part of the cycle for it to collapse far enough to bottom out. but-------at some certain rpm rates the lifters spend more time ( mili,mili micro sec) that total collapse is possible where at high rpm ( compare 600 rpm to 6000 rpm---10x ???) and add in the oil flow rates etc etc---------

Also-being an used older block that has been rebuilt-which brings up countless areas where clearances have been disregarded ( mainly $$$$$$$$$) in the rebuild process-------if a flat top piston is put on the rod backwrds, it will double the amount of normal wrist pin offset-----some HP forged pistons won't have any offset and are noisy at some idle range rpm with lighter oils---I'll try to look up the specs on the wrist pin offset so I can maybe explain this line of thought----