oil pump failed

Its possible you had 30 ish when the engine was cold. I have never seen a small block take out a pump like that and you still had 10psi. I bought a 340 dart with a new 340 that had no oil pressure and needed to be re-rebuilt but it was missing the galley plug in front if the distributor gear but you mentioned you checked for that I think. it always had 10-15 psi hot and 60 when fixed. Good luck and I am interested to see what you find.