There are many possible causes, a few could be: too much clearance on the bearings on the mains or rods, bad oil pump or worn housing, possibly an internal fitting not installed in a galley port after case overhaul.
I'm no expert but I did overhaul my 320. None of these will be an easy fix.
I'd put a direct reading old style mechanical guage on there and retry it first before taking anything apart. A ground run should tell you with a direct guage. An oil line direct connection type. The pressure port is on the right aft upper side of the engine normally. You should also have a temp guage installed,,,very high oil temps will drop the pressure dramatically.
 
First off make sure you have good data. Fix your oil temp gauge and verify the oil pressure reading is good and we'll go from there.
 
Randy,
Considering whats been done and how this things been put together. I would consider dropping the sump. pulling off the accessory housing and checking the back of the case to make sure you have all the oil galley plugs installed. And while you have the accessory case off pull the oil pump cover off and see what you have for impellers and housing. It sounds like you have an open port somewhere.
Ryan
 
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