Thanks. I hope to be able to supply you with enough info to drive your truck back... tow truck rides get spendy.
This was the best I could do for a wiring diagram of the system and it's similar.... just not the same as your '75. The dual-ballast system went out of production some around the late 70s, so this one reflects the single type.
The ECU will be the same, however... as will be the pinout on the unit. Some ECUs are equipped with a 5-wire connector and I'm not sure what the 5th wire would do since everything you'd need to produce spark is shown here on the 4 wire setup.
The wire at about the 1:00 position is your coil negative circuit, the one that actually grounds and controls the process of spark production.
At 3 O'clock is the ignition power feed, which will come through your ballast resistor. More on that later.
Following around at 5 and 7 O'clock are the distributor pickup plate wiring circuits.
Grounding for the ECU is proved by the mounting bolts to the firewall. It's quite possible that the ground is weak or lost, so don't hesitate to shine up the connections or at least test the case for continuity to ground or for residual voltage (should show 0v).
Start by checking for voltage at your coil with the key turned on. Since power is being fed through a resistor, you'll see less than battery voltage... probably something around 8v or so. The negative side of the coil should show very little voltage because the ECU provides a ground circuit to the coil at all times with the key on unless it sees a peaked AC signal from the distributor pickup plate at which time it releases the circuit, allowing the "saturated" coil's magnetic field to collapse and produce spark. It operates on a 44 degree dwell cycle with the engine running if you remember your old points-n-condenser stuff. The coil will begin to feel warm to the touch after a few minutes of sitting with the key on.
With a dual ballast resistor setup, the ignition switch supplies an alternate path for voltage supply to the coil at the same time as you're cranking the engine to increase voltage that naturally drops with starter draw. Without this voltage shunt, you'll have a tendency to have difficulty starting the engine and it might fire only as you let off the starter.
What I'm not clear about here is just where that voltage source comes from. Later years took a feed from the starter relay... something that became energized at the same time as the starter solenoid. For now, check the feed side of the dial ballast at both levels to be sure you get power fed to the unit. Then check to see that the ballast hasn't burned out internally, especially on the bypass side. If ohmed out, expect to see something like 1 or 2 ohms of resistance on either resistor, which runs horizontally to the unit.
While the engine will run with straight 12v wired to the (+) side of the coil, the coil and ECU may overheat and fail after a bit of run time. If you have to do some jury-rigging with the engine wiring to drive it that way, give it a few minutes to cool about every 15 minutes or so.
Next up would be the distributor pickup plate if you still have problems with producing spark. I believe your unit will be a single pickup, but duals happened in some years. The run side pickup plate will have a connector shaped like the one I have highlighted above, where the start side pickup (if equipped) will have two bare male terminals on one end and two females on the other.
With the key on at this point, pull the coil wire out of the distributor cap and place it within 1/2" of a good ground. Then disconnect the pickup plate connector that looks like the one I highlighted.
Locate the bare terminal on the harness side connector and touch or scratch it to a good ground source. The coil mounting bracket works well for this. It should produce a storm of spark if the circuit to the ECU is good from that point. If this is the only way that you can produce spark, it pretty much means you're lacking a signal from the distributor.
Make sure the distributor rotor turns first, then check your reluctor-to-pickup plate gap or replace the pickup plate. Either operation pretty much requires removal of the distributor by the way.
Instead of sweating the .006" air gap that's specified for this adjustment, I found a more practical method. Since the distributor shaft bushings will be worn (slop) and the shaft will almost certainly have some wobble to it, my preference is to set the air gap as close as is possible while pressing the distributor shaft toward the pickup plate. The important things here are to get it close as possible... but have no contact at any of the 8 reluctor tabs as the shaft rotates. When that's achieved... you should be all good.
I'll stop there for now, except to recommend taking some extra wire for jumpering purposes. You're gonna be a long way from Walmart and spare wire just plain comes in handy.
Good luck and we'll talk in a bit!