I also was surprised to see the low oil pressure.
The
3/8ths plug missing explains why.
We had just the opposite problem.
We built a full pressure, drilled B block in
1976 with oil system mods to oil galley on side cover following Dan Iandola's Rod and Custom article.
Stock B Oil pump was opened up following Dan's directions. Shaft was turned also because we used a Model A shaft.
On first fire up the 3/8 inch thick aluminum side cover blew the gasket and oil dumped out on ground.
After adding a number of 10 32 cap screws between each cover bolt it held the gasket at idle.
Oil pressure was 100+ PSI with engine at above idle. I do not remember the RPM.
The camshaft moved in/out also!
After welding a pressure bypass valve cut from a V8 oil pump to the side of Model A oil pump we got it down to 30 or 35 lbs.