It's been a long time so bear with me.

The injectors actually use a reduced voltage, something like 5-7 volts. You can probably check this with a volt meter however because of the pulse, the reading may fluctuate. You can also crank/run the engine and see the pulse with a regular test light, however it will be dim of course.

I never saw a TBI regulator unit go bad. I have seen the IAC motors gum up and the port within the TBI clog with carbon. Sometimes the entire bottom of the unit can clog up, but you would have definitely noticed this if you had it off for the motor swap.

If your injectors are pulsing but no fuel can be seen, it's a pump problem. As long as you can hear the pump run, the oil pressure switch is working, letting it have power.

I doubt it's injector(s). These are also tough as nails and the engine will run on one.

I still have my GMC electrical/driveability FSM. Are you setting any codes? I can run you through trouble shooting the code/problem or scan and e-mail you the pages if necessary. Let me know if you need anything else.

I would go through and re-check all of your connections, even at the PCM. I bet it's a bad connection or sensor going bad.