What does everyone think? Had a stock 330hp 454 put in a melling high volume oil pump had a steady oil psi of 60 hot (electric) gauge. Built the motor everything a 420 clone put in a new melling high volume oil pump mech gauge reads 50 psi hot after a hard run 5100 for maybe 2 min oil psi goes down to 42psi back to cruise 3500 back up to 48-50psi. Running 30 wt oil. How come a old used motor would carry better oil psi?

Sounds like your running good #'s to me. Pumps and bearings have diiferent clearances' , etc.. Mech gauges and electric gauges never read the same. If you want more pressure go to thicker oil.

Bearing clearance.

Pressure's not terrible. Bearing clearances are probably the culprit. Heavier oil or larger oil cooler would help.

ditto, I'd run heavier oil