Will not start after replacing head gaskets

Have you tried a bit of starting fluid to see if it will fire on that? That would at least confirm it will run and is in time. Have seen a few wrong timing marks on those belts.
What are your compression numbers?

Check that you have power to the injectors. One wire (red with green stripe) should be battery voltage with the key on. The other wire is grounded by the ECM as it fires the injectors. A shorted injector could pull the voltage down.

If no power check that the ECM relay in the fuse/relay box is activating and that the ECM fuse near it and fuse 8 (dash box) are both okay.

Was that with all of the injectors unplugged and then testing the wire connectors or using back probing with them connected?

The injectors on that rig are paired on the ground side and fired that way by the ECM. So what you saw would be normal as the voltage would go through the injector coil and out the "ground" wire then back to the one you were testing. However it did show that you have voltage at least.
The earlier noid light test is starting to point toward the ECM controlled grounds as being the problem but it could be fooled the same way because of the loop.

I would follow the harness back to a connector rather than pulling the intake. Then test there on the three ground control wires.

The way that engine is set up it has one wire (red/green) that feeds battery voltage to all the injectors as well as to the ECM and other components. With the key ON that wire should have 12 volts. The injectors are simply a coil of wire inside so if you feed 12 volts into one side it will simply feed through the other wires connected to them. With everything connected up you should be able to gently backprobe the ECM connector with the key on and find battery power at each of the 4 wire connections for the injectors, That would be pins 11 and 14 in connector C and pins 15 and 16 in connector D. If that checks OK, Shut off the key and the relay that supplies power to the injectors should open. The voltage on those pins should now be gone.

The next part of the test involves unplugging all the injectors from the harness. Now attach a test light to battery positive and have someone else crank the engine over. While it is cranking, put the test light probe into ECM control wire for each injector. The test light should pulse if the ECM is trying to fire the injector. If you get no pulse at any wire the ECM is not trying to fire the injectors. If the light comes on bright at any wire and doesn't flash the ECM is faulty.

The ECM won't try to fire the injectors if there is an issue with either the crankshaft or camshaft sensors so don't condemn the ECM before you test those.

If you still have the old one, plug it in and try it. The crank sensor on that feeds into the electronic ignition unit as well. It is possible it has a problem and isn't passing the signal through to the ECM to trigger the injectors.
To test the crank sensor wiring you can disconnect it at the ignition module (should be a 3 wire connector) The outer 2 (1&3) are the crank sensor feeds, the center is a grounded shield. Disconnect the other end at the sensor and jumper the two pins. Connect a continuity tester up (test light to batt, tip to one wire, other pin to ground should complete the circuit) You should show a good loop, then wiggle, bend and see if the light stays on through it all. If it does the wires are OK. Connect the grounded wire to the center pin and move things again, just in case the shield is damaged and grounding the signal.