That much hot pressure drop is too much, especially with straight 30 oil. Drop the oil pan and do some Plastigauge checking- - - -I'm going to bet on excessive bearing clearance somewhere, or possibly a weak pressure relief spring in the oil pump. Another trick we used to use frequently was to rig an oil pump with a flex line output in a 5-gallon pail of oil, run it with a drill motor, and pressurize the oil system through the pressure port on the side. Watch the leakage at the rod, main, and cam bearings with the oil pan removed and a big catch pan under the engine. It will be pretty easy to spot where the major pressure loss is coming from.
Jerry
