331 with fuel pressure issue

you either have a serious leak somewhere or a pinched feed line to have pressure that low

The stock fuel pump (88 LPH) is too small for your current engine. You need a 155LPH or larger pump to match the engine HP and injectors.

Fuel pump pressure test
Disconnect the larger of the two fuel lines up by the Schrader valve. It is the return line and does not have the Schrader valve on it. Find a piece of rubber fuel hose and clamp it on the return line
coming from the regulator. Stick a bolt in the other end of the hose and clamp it. Make sure that
all your connections are tight and leak proof as possible. Be extremely careful that your
test connections have no leaks. There is a definite fire hazard when you do this test.
When this powers up, you don't want fuel squirting everywhere.
Hook up the fuel pressure test gauge to the Schrader valve pressure test port.

Turn the ignition switch on and watch for leaks. You may want to use a helper inside the car
to cut the switch off quickly if you have a leak. To trick the fuel pump into running, find the ECC test connector and jump the connector in the Upper RH corner to ground.
.
Caution!!! You have blocked the return line for the fuel pump! Pressure will rise very quickly past safe levels with a good pump

If the pressure goes up past 55 PSI, the pump is good and the fuel pressure regulator is bad.
If the fuel pressure does not hit 55PSI or more in a few seconds, the pump is bad or you have electrical problems.

It should not run on 5-15 PSI. I would use another gauge to be sure the reading is accurate.