I have a 440 that I had rebuilt by a local guy...now a 505.
Other modifications from stock:
It's got a hydraulic roller cam with COMP lifters and PRW 1.6 bushed (not roller) rockers.
I put a new, standard (not high volume or high pressure) oil pump on it.

WHen it's cold, it's got about 50psi at idle, which does NOT change as you rev the motor.
After it's all warmed up and I've driven it around the block a few times, it's got 15psi at idle and will go up to about 40 psi by about 2500 rpm and then go no higher with higher RPMs.

I bought a cheap mechanical gauge to double check my electric gauge, and am getting identical numbers.
I also shimmed up the pressure relief spring and it made no difference.

What else should I be doing or checking before I pull the engine out to check the bearing clearances (which I am hoping is not the problem)?

You need a high volume pump. Some will say that's a Band-Aid but if it improves the pressure.............

Before taking it apart? Cut the oil filter open for inspection. Oil an ultra light viscosity? HV pump. Ask the guy what clearances he used.

I agree, but we don't know the clearances or what grade oil the OP is using.

It would be good to know that.

Try a Melling HV pump.

First ask the builder for the clearance specs
then cut the filter open and see whats in there
then I would ask what oil is in it... if its
10W30 or better put a high volume pump on it..
if it has 10w40 in it you should see 70 psi
priming it with a 1/2" drill motor...super loose
clearances will show low pressure.. make sure
the pressure relief valve is tight

I am using 10/30 oil. Forgot to mention that. I know it's light, but wasn't sure if that alone would explain such low psi.
He said that 'it's pretty tight' and that I should start with 10/30. I have a sheet with the numbers on it somewhere, but I can't find it at the moment. But they were pretty tight and I want to say they were in the .002x range.

One thing I noticed about the rockers, is that when I primed the oil pump, I had the oil filler cap off. Oil from the rocker right under that hole went SHOOTING waaayy out over the fender and on the ground. never saw anything like that on any other motors I ave primed, but have never built a motor with shaft mounted rockers before.

Come to think of it, that is one more thing I think I need to check...pull the oil cap with the engine running and see if there is a crazy amount of oil coming out.

Rocker shafts should have timed oiling from the #4 cam bearing. If someone grooved the bearing or cam journal, you might have too much oil feeding the shafts. Might need block restrictors.

I should mention that I got a short block from the builder. I put the valve train in. So if there are any rooky mistakes that could be made, please let me know because I could have made them!

Thanks for the help, guys.

Did you put the rocker shaft on with the oil
holes facing downward.. thats the load side
and the way the factory does it.. there is a
direction the oils should face but I dont know
for a BB if thats inward or outward on the bottom
but on factory shafts the "notch" was facing forward
with the holes down.. the notch is on the front of the
shafts them self... but it sounds like the flow is there
to spray across the fender.. I've never had that happen
even with 70psi so the shafts could be on wrong

if it's a roller cam is it likely the cam journal is grooved for full time oil to the rockers??

the problem is right there. undo it & see what's up. Keep us posted.

The first thing to try is easy and free, you have nothing to lose and it often solves the issue. Add a washer under the oil pressure relief spring in the oil pump cover.

OK, I am going to flip the shafts and see what happens.

I was also thinking of just putting the factory shafts and rockers on temporarily to just see if there is an improvement.
The cam has .550 lift and I have beehive springs, so I think they should work in the short term and low rpms? Just to eliminate one variable...

Yeah, I tried that (actually added a few just to see what would happen.) It did not seem to make any difference.

If you shimmed the pressure relief and the pressure didn't go up with RPM there is a oil gallery plug missing, or some other giant leak.

Not that I recall.

Good plan, I would drill prime it (CCW) as is right now with a variable speed drill/5/16 hex rod with the valve covers off & no way you wont see where the leak is with a geyser of that magnitude.

This is what I'm starting to think

I doubt he'd see 40 psi with a gallery plug out using a stock pump. That's a gigantic leak! But yes, some leakage somewhere.