Gotta love it when you doubt something and it turns out to be true! I also bought a truck with a "rebuilt" motor several years ago that had low oil pressure when i got it running

I did a compression check and it was good, so I pulled the pan off to find the oil pump pick up screen almot completely plugged from silicone that had been floating around and sucked up. The PO used WAY TO MUCH sealer on the gaskests when he assembled the motor. Just something to think about. Good Luck.
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67 short wide 355ci 3 on tree
69 short wide 250ci 3 on tree
68 long wide 283 3 on tree
72 2wd Blazer project 454 5 speed