troubleshooting oil pressure

I have read many oil pressure threads and tried a few things with no luck, but at least I ruled some things out already.

The motor: 74 360 .40 10.5:1 J heads about 400 miles on fresh rebuild with 2 thousands bearing clearances (told by machinist, and I checked with plastigauge).

Symptoms with Mopar Performance HV oil pump and old mechanical/hydraulic gauge:

50psi cold oil pressure with old mechanical/hydraulic gauge mounted by the distributor. When motor is between 180-200 degrees city/HWY the oil pressure was at 20 psi. And at idle it would peg 0 psi on the gauge. I could use the throttle to keep it up around 20 psi. I didn't continue to run the motor like this.

Symptoms with Melling standard volume oil pump and brand new mechanical/hydraulic gauge:

75psi cold oil pressure, motor between 180-200 degrees pressure grows and drops as rpm goes up and down. Stays between 20psi and 40psi depending on rpm. 5-10 psi low using the scale of 10psi per 1000 rpm. Hot idle psi drops to 10- 15psi.

What I have checked so far is that there is an (rear main) oil galley plug and it is roughly 1/8th" deeper set than my 340 builder block which still has that plug in place. So I believe I can rule that out. The intermediate shaft is in good working condition with no rounded edges where it should meet the pump. The other things I have done were change the oil pump to stock melling from HV mopar pump as well as a brand new gauge, which increased cold psi by 25psi but low psi at operating temp still remains an issue.

Any tips on what I should check next? I am used to Pre-rebuild having consistent 50psi at operating temp, which leaves me concerned about my present oil pressure. Thank You in advance!