Won't Fire - Need Help!

First, "suspect the gauge." If this is an electric gauge, get a mechanical. No need for a dash mount gauge, you can buy a generic test gauge, or even use your compression tester with the proper adapters / fittings.

The big tricks that I'm aware of on SB pump pressure, other than "standard" issues like badly installed, damaged, wrong size bearings or journals, 'effed up pump bypass or filter problems

Issues with pan to pump interferance. Check carefully. I believe we had one member break a pump

Issues with ARP (and other ?) studs with pump fit. This may be the one I was thinking broke the pump

There's a drive in core plug in the vertical oil hole from the rear main going up to the galley area

There's a recessed plug FORWARD of an access plug in the rear of the engine. I believe you can see this by pulling the dist. The plug is down in the rear near the distributor gear facing to the rear. These are threaded plugs

I'm not too sure the front plugs are an issue. There's documented proof of guys having engines with no plugs, and at least one claimed the factory never put them in some engines