"Jumping the right side of fuse 4 to the left side of fuse 6 did nothing - no pump buzz.
Hooked up a spare ECT (just let it sit in the engine bay) - no change

That last note is probably the culprit (main fuel pump) - the pump is original (347,000 miles), but I just wanted to see if anyone had other advice. It seems like the car is running super lean."

HillBilly explained the wiring at fuse 4. Your test really begs the question asked: did you do this with good continuity through fuse 4? In other words, what you did should have had both pumps buzzing, but none would buzz if BOTH fuse 4 were open and the tank pump were dead.

Thing is, the tank pump can be dead for years and you won't know it, if you keep the tank fairly well filled and are not driving in the hills in July. You know that.

Anyway, I believe changing the main pump on mileage or suspicion is a mistake. That's just me. See In the Tank again. But maybe it is time for you.

If you substitute the ECT like you mention, you need to provide a good ground between its body (threads) and the head. It is a three-terminal device.

My first suspect given last night's weather would be ignition despite your assertion the cap 'n' wires were in good shape. How about the power stage connector?

Basic check has you getting a can of starting fluid, so you can prove the problem is fuel if that's what you suspect. If you do then, maybe you could devise a pressure gauge connection like many of us have, or in most cases a squeeze of the return line with the pumps enabled will tell you the rail pressure is up.

The 1990 wiring: The left side of fuse 6 is powered separately from the 25A fuse under the hood, so if you use that as a power source, you are bypassing a possible red wire/blade fuse trouble.

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Art Benstein near Baltimore

If you're riding' ahead of the herd, take a look back every now and then to make sure it's still there.