I don't have any extra gauges that I can plumb in. I have a car until tomorrow night so I could drive out myself to borrow them.

But I'm not sure how a bad pump could cause this issue.. the gauge certainly seems to be working. Even if the pump was all over the place, the regulator should be able to hold reasonably constant fuel pressure. The fact that it can't suggest to me that the problem has to be at or after it...

Edit: When I searched around about this, I saw many people mentioning that they had to drill out a portion of the fuel return on the 2g sending unit when they went to a Walbro 255 with rewire. The fact that this setup ran fine for 2 years leaned me away from it; the fact that the return going into the tank seemed unrestricted pushed it right out of my mind. But I'm wondering if there's dirt or something in there plugging things up.

I'll probably pull my sending unit out for inspection after trying a few other tests..