The reason I went after the fuel filter is based on the removal of the fuel line at the regulator and no gas, same idea with the check valve. The filter is also cheap and easy to replace and rule out, especially if you don't know how old the current one is. You have spark, and running pumps, so ECU and relay seem to be ok. Here is the longshot: the entire in-tank assembly, not just the pre-pump. I had a 78 GT that stumbled and ran like crap with a new pre-pump and main pump, ignition parts, and most of the fuel system. Just for fun, I pulled an in tank assembly (fuel sender, pre-pump, pick up tube) from the junkyard and swapped, car ran like new again. To this day I can not explain why this happened, I can only guess that there was a bad connection somewhere in the wiring to the pre-pump, maybe it sent a bad signal to the ECU? Like I said, a longshot.

If you give it a shot of starting fluid, will it start?

The stumbling before it died may rule this out, but how about the timing belt? Pull the oil cap and watch the cam as someone cranks it over. Maybe the timing belt jumped a few teeth even?

One last longshot, plugged cat?

Keep us posted, and good luck.

John