Can I bypass ECU to fuel injectors?

Okay, so, you're asking why the injectors are not getting pulsed? And what year ECM are you working with now? The 1992 or 1988'? And is the 1988 from a G-30 van as well? From the diagrams for the 92' the pickup coil in the distributor is where the timing signal comes from, for example newer vehicles use a crankshaft position sensor to reference timing and then sometimes a cam sensor to reference the #1 cylinder's TDC. So, in a distributor sets up the pickup coil or coils do the same thing. That will be the reference for spark timing and injector pulse.
I did find wiring diagrams for a Chevy G-30 van, 5.0l and 5.7l and actually these diagrams are a lot better than the 1992' ones.
And to check for the timing reference you can put a test light attached to Battery Positive and while cranking the engine touch the test light to the negative side of the Ignition Coil. The ignition coil has 2 circuits built into it. The 2 wires attached to the coil are battery positive and ground. That is the ignition coil's Primary circuit, when the ECM pulses the Ground side (or Control side) of the coil, this induces high voltage into the coils Secondary circuit which the spark plug wire is connected to. So, with a test light attached to battery positive and touching the negative (Ground/Control) side of the coil, your test light will flash, because the ECM is continuously turning the control side off and on to induce the high voltage into the Secondary circuit.
You notice that older models have that wire running to the distributor that has a connector you unplug to set base timing with a timing light. That is the EST circuit. It's the ECMs control of spark timing to the Distributor. When it's unplugged the ECM no longer has control of ignition timing. The ignition control module has control of spark timing then.

These diagrams are the ECM from a 1988 Chev G-30 5.0 liter. Notice on diagram 4 the Pick-up coil and the Ignition Coil + and C(control). This has a separate Electronic Spark Control Module. A common GM setup back then. There are also break down wiring diagrams of each system, Fuel Control and Idle Air Control- Ignition System- Throttle Body Injection (TBI).
5th Diagram is the Fuel Control.

No, there are no wiring diagrams for that at all. There's a vacuum line diagram, but it's almost impossible to see anything on it, it's so old.

It's all good, that's why we're here. To help people out. As for the ECM, I can't really identify an ECM by is serial number. I'd need to know what the ECM is designed for. Is it for the 1992 G-30 5.7 liter? I did a search on that number for ECMs and came up with a C1500 Chevrolet ECM 4.3L, 5.0L, 5.7L, 1988, 89, 90, 91'.
But just so you know, the engine that will be going with this ECM, needs to have the correct number of teeth on the crankshaft reluctor wheel. Or the correct pick in the distributor if that's what it has for a setup. The ECM is expecting to count a certain number of pulses from the crankshaft position sensor, or it will not trigger the ignition and/or fuel injectors correctly.
So, I looked up a 1991 Chevrolet C1500 5.0liter, and there are some different options, for example there are diagrams for just the ECM and sensors, then there are diagrams which are more like the whole harness type of diagram. Also, there is listed ECM 1 and ECM 2 but I don't see much difference in the two ECM diagrams. I'll give an example of each one below.
So, pages 1 and 2 are just the ECM by the number you gave. Pages 3 and 4 are just the computer and sensors. No bulk connectors or anything like that, Pages 5 and 6 are the Engine Harness type diagrams.

The fuel pressure regulator on the fuel rail should be vacuum operated. It actually only really increases the fuel pressure by about 5-8PSI. That computer though is going to need a pulse from the pickup coil anyway to fire the ignition coil. And are you only referring to fuel pressure or line pressure in the transmission? The diagrams that are here for that computer also have transmission input pages 1 and 2 and output pages 3 and 4.
But I think you're trying to throw too much together that in the end you're going to end up with more issues to deal with than you want. Really putting on a throttle body that has two fuel injectors will actually be easier since that computer is already set up for that.