I have installed this system on my 52 B3B, however I am running an aftermarket wiring harness, but I am running a Mopar electronic ignition system. I will have to look and see where I have those wires running, but I will say that all EFI systems are very sensitive to input power, it has to be clean and strong. I believe you are correct when you say that the wires going to the ballast resistor are wrong. I also have a Holley/F&B system in my 70 challenger which has a factory harness and in that system I had to do away with the ballast resister.
As for the tach, it needs a signal and if you are running a Mopar system, I would believe the positive side of the coil would be a source but I would have to research it (I will look at my truck).
Pushing the gas pedal only allows engine vacuum to draw fuel into the cylinders as there is not accelerator pump, and having the wiring incorrect allowed sporadic firing which resulted in your backfires and fires.