I generally see 5-8 psi more with 20/50 than 10/40 but my climate is mild. 55-85 degrees, I don't drive my car much at 95-100 degrees but ambient temp makes a difference.
Try a mechanical gauge for kicks. It picks up at the block where the
oil line adapter is. If it is still low you may have a bad
oil bypass spring in the pump or in the block.
Was this an engine you owned prior to the rebiuld? You say the
oil pump is almost new??? What does that mean? Was it on the engine before with low miles to prolong the life of the engine before the build? or was is something someone had lying around. It could be the pump. Are you sure it is a HV? What was the oil pressure on this engine with this pump before the build (did you have good pressure with this pump before the build)
I had similar problems with a standard pump that was on my car when I bought it. It could be the bypass spring in the block was improperly reinstalled if it has one (not all c/o's do) it is behind the fly wheel above the cam freeze plug.
This is a solid cam engine now, I will assume this as you indicated the rocker arms are adjustable.
Was this a solid cam engine before it was built?? If it was a hydraulic lifter engine and if the machine shop didn't plug the oil galley to the lifters that could be your problem. The lifter design for the hydraulic lifters takes into account the need to maintain pressure, solid lifters are the opposite. they are designed to scavenge oil off the valley pan and will allow oil to flow around them freely. This isn't something the machine shop will know or care about (they should, but may not) especially if you bought your heads elswhere and only had them do the short block???
I hope this helps.