Rebuilt engine woes

This is not my first engine build by a long shot but the troubles I'm having are ridiculous. I picked up a really clean 02 D2 that apparently suffered a second headgasket failure. This used engine was acquired from Will and installed by Drew about a year earlier prior to the failure according to the owner. The heads were skimmed and pressure tested. New ering gaskets and headbolts were installed. Afterwards, I noticed an incessant tick at idle but no oil light flicker. I rebuilt the upper valvetrain with new components and put in a new 4.6 cam/lifters. The tick persisted as did low oil pressure (viewed using a mechanical gauge). Breaking it down I also noted 2 wiped cam lobes. I had a completed D1 short block in the garage so I moved all the new stuff over to this engine (changing out the CKP sensor bracket), and installed a new camshaft/lifters and new front oil pump/timing cover. The upper engine was primed with oil but I didn't open the oil pump and pack with petroleum jelly. When I fired up the engine yesterday afternoon it clattered like crazy momentarily until oil pressure built to 50 psi at 3k rpm which steadily dropped as time went by. After 20 minutes of break-in I dropped it to idle. Oil pressure became nonexistent (mechanical gauge installed) and the oil light flickered. There was a light knocking sound which I thought was a rod. It was shut down and I walked away very disheartened. A couple of hours later I started the engine and it sounded much better. There was no knocking and oil pressure was 25 psi at fast idle but dropped to 0 as it warmed up. The tapping noise returned but at lower db's. I haven't decided what direction to go in now but think I'll have to pull the engine yet again. Can anyone offer up what I missed here? The cost and frustration are wearing me down.