Perry....let's get some basic questions resolved first....what rocker arms are you using? Have you done any restricting to them either in the block (two passages) or in the heads between 3 &4 and 5&6 cylinders? Do you have a hydraulic or solid style camshaft? What did you use for a side oiler relief spring....did you install the threaded oil galley plug that connects to the very front lifter bore on the drivers side (#5) ? this is installed from the front of the block prior to installing the distributor? Your getting oil to the rockers so I assume you installed the side oiler cam bearings vs the regular FE cam bearings and the holes are lined up correctly?
Let's get these questions answered one at a time and then move to the next ones to help resolve what you believe to be low oil pressure....20# at idle is not necessarily low and it certainly depends on several possible factors...I assume you have a mechanical gauge too...that worked with the previous engine with different results....
George