Hope YOU..( and I) find what the problem is, cause I've got the same problem with my 428!
All New, lots of work done to the block. Passages opened from the pump to the filter adaptor, to the mains, and the mains matched to the Bearings! Also a HV 57 Melling pump. Comp Cams Extreme Energy Hyd. Cam Kit.. Erson Kneedle bearing rockers.. Restricted the passages in the heads at .090" to start.. LOTs of oil to the top when priming! Tried .060" and then.047".I started the motor... Never reachd 60psi and when hot below 20!! After it's hot and I start it and let it idle, it hardly gets off the 0!! only to about 10 and then only to around 15 to 20psi.. What's up HELP needed here! Ray