So I finally got a decent fuel pressure gauge installed with no leaks and was able to monitor until it stalled and died. This is after I went ahead and pulled out the tank, which I've been meaning to do for years anyway. No rust, varnish or sludge, although the return banjo had some gunk lodged in the outlet. Checked all of the fittings, hoses, relief valve, etc. Purged the hard lines with carb cleaner and a few blasts of dry shop air. Tested current to the fuel pump, which was right at 12.8 volts continuously. All good.
After a couple of hard starts to get the gas flowing back into the engine, the car idled fine for a while with fuel pressure right at ~35psi. Then it started to drop rather quickly, maybe over about 30 seconds, to around 20 psi, whereupon it stalled and died. So I started it again, and the pressure would only increase if I pulled the throttle, before dropping again. I tried pinching off the return hose from the regulator with a pair of vice grips, with no noticeable change. Stall.
The fuel pump is new, after the original one seized after a prolonged stay in the garage. But what else could it be? I swapped three different combo-relays with no difference. Spark is strong. Battery is new. Current to the pump is full strength and consistent. Fuel filter is new but I'm going to swap one from the parts shelf, just to see what happens. Maybe it's waterlogged.
I could be wrong, but I think this is classic pressure regulator malfunction, either from a leaking diaphragm, broken spring, or some other internal problem. Because if you give it throttle like you did, it will demand more fuel, which will drop the pressure on the regulator outlet momentarily. It should catch up quickly though, if it's ok. Pinching off the return hose may not have any effect in this case... depends on how the regulator is failing.
So what's the story on the pressure regulator? Is there reason to suspect it in your opinion? You could have a bad pump... we've seen defective new parts before, but it's not my primary suspect at this point.
Good job on getting a proper fuel pressure gauge hooked up and checking inside the fuel tank. I take it that the in-tank strainer was clear, then? A gradually clogging strainer or filter could cause your symptoms, so a quick filter replacement is a good idea to eliminate that variable . . . .
However, based on your description, your symptoms smack of a failing fuel pump, not a bad fuel pressure regulator. Did you pinch off the fuel return line as the engine was stumbling, or during normal running with good fuel pressure? With a healthy fuel pump, completely pinching off the return line should cause the fuel pressure to go sky-high (like 80+ PSI; this is known as "dead-heading" the fuel pump and is a good stress test).
The fuel pressure regulator is designed to vary the fuel pressure in relation to intake manifold vacuum to maintain uniform fuel delivery volume at the injector tips (which are of course inside the intake manifold) regardless of throttle position. Increased vacuum at the fuel pressure regulator pulls the diaphragm against spring pressure, which opens up the internal chamber and allows more fuel to return to the tank, thus decreasing fuel pressure in the rail.
With the engine idling normally and with good fuel pressure, snap the throttle open several times---snapping the throttle causes intake manifold vacuum to momentarily increase, which with a properly functioning fuel pressure regulator will result in a commensurate decrease in fuel pressure before returning to normal. Slowly opening the throttle to WOT will cause the fuel pressure to slowly increase (up to a point), as the open throttle plate will decrease intake manifold vacuum accordingly. Manipulate the throttle in different ways while watching the fuel pressure gauge---if it reacts as described, then your fuel pressure regulator is probably OK.
Also, you mention that "current" to the fuel pump is good at 12.8V. This is not an accurate statement as "current" refers to electrical flow, which is measured in amps, not volts ("voltage" is electrical pressure). When a fuel pump fails, it typically draws an increasing
amount of current (due to increased internal resistance) while voltage to the pump (which is what it sounds like you are measuring) remains constant. You would really need an inductive amp clamp and a scope to accurately measure current to the pump, which is usually not available to the average DIYer . . . .
Repeat the fuel pump "dead head" test and re-check the fuel pressure vs throttle position and check back with us.