Junkyard 5.9L Magnum + Edelbrock LA heads build
I recall Yellow Rose discussing galled pushrod cups at high RPM's with rockers like the 273's, oiling via the standard LA top-end oiling system. I very much suspect that the stock top end oiling restriction... that of the cam hole lining up to allow oil flow for <10 degrees of cam rotation once per revolution...... is the culprit. When the RPM's get high, the time window of these openings gets so short that the oil's mass in the passages just can't get accelerated adequately during that short time window, and this is made worse by the long length of that oiling path. So the top end starves more and more as RPM's go up.
Seems like the direct oiling via pushrods would solve that. Your approach would also help as there is nothing to limit/stop the flow to the rocker shafts at higher RPM's; it ought to stay constant as long as the pressure is constant.