1998 Chevy 2500 2WD truck

Im wondering where you connected your external gas supply? I see an inline filter. Have you eliminated that? I know if it is partially clogged it might be limited flow vs. Limiting pressure. Also in reading your post, I started thinking about back pressure. I would check the fuel vent pipes at the tank. Kinks, cloggs, cracks. They shiould just be nylon pipes, and once kinked, they need to be replaced.

Good luck, and please let us know what you find out.

Ok, if it only works with fuel poured in and you have good pressure, I would rule out the regulator, and fuel pump. Need to know which engine you have.I think there are 5 different ones for that model.

It sounds to me like your injectors are not opening. In most of your type of vehicles power is pupplied to all injectors by the underhood fuse-relay center. Usually the fuse is a 20a ECM 1 fuse. Make sure that is getting power first.
That eliminates power to injectors as a problem, and just leaves control.
Control is done by the VCM (Vehicle Control Module).
In most vehicles, once the ignition signal is satified from the ignition switch, it tells the VCM to give fire and fuel. Since you have ignition, we can assume that signal is getting there. Now comes the tricky part.A timing signal is sent to the VCM from both the crankshaft sensor and the cam shaft sensor to time the fire and fuel. A lot of vehicles will use the crankshaft sensor for spark, and the cam shaft sensor for fuel injectors. Im trying to read up on your truck and see which your uses.
I think most injectors run on a 5v supply, so I wouldnt put too much voltage to them.
I did find these descriptions I will paste for you to read.
STARTING MODE
When the ignition switch is turned to the ON position, before engaging starter, the Control Module energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The Control Module then checks the Engine Coolant Temperature (ECT) sensor and Throttle Position (TP) sensor in order to determine the proper air and fuel ratio for starting. The Control Module controls the amount of fuel delivered in the starting mode by changing how long the injectors are energized. This is done by pulsing the injectors for very short times.

Well, if your injectors are getting power, and you have fuel to the Fuel meter body assembly, then it makes sense that something in the fuel meter body is obstructing the flow. Im not sure what you meant by Briefly lite up the light when that cylinder was supposed to fire.
Could the fuel regulator be blocking the flow.I doubt it. It should just be a spring loaded relief valve that opens to keep the pressure at or below 66 psi.
My gut felling so far is that the timing for your fuel injectors is not being signaled correctly, or at all. I cant see all the injectors being clogged at once.
Without being able to connect test equipment and see if the injectors are getting the proper signal, it is a little hard to trouble shoot.
I have had cars not firing at all and it be a crank or cam sensor, but I hate to steer you towards something without knowing for sure.