low oil pressure
I hope that, during your rebuild, you checked the engine bearing clearances. Green plasticgage has always worked for me, and is an inexpensive way to do it. As I'm sure you know, main and cam bearings out of spec will result in low oil pressure, and yours sounds extremely low. I also hope that bearings aren't the problem, since you've already installed the engine. Good luck.
I hope that, during your rebuild, you checked the engine bearing clearances. Green plasticgage has always worked for me, and is an inexpensive way to do it. As I'm sure you know, main and cam bearings out of spec will result in low oil pressure, and yours sounds extremely low. I also hope that bearings aren't the problem, since you've already installed the engine. Good luck.