fast efi problem

I don't know about wideband. Many of the other sensors can be substituted with resistors, temp, etc. As you discovered, IAC, etc, can be unhooked

One you cannot live without is throttle. On my Holley, once you turn off "learning" in the ECU, if the O2 goes bad or you have some other reason, you can unhook it and it still runs fairly well

No knowing what you've discussed, let's talk about ground and even power loops.

Your battery is trunk mounted? Where exactly is the power and ground mounted? On my Holley, they do mention "single point" grounding.

Also, some signals, and MSD especially warns against this, ------some signals can "crosstalk"

This is no joke with high output ignitions like MSD. That stuff is just like lightning.......a huge powerful pulse of energy, which can induce pulsed energy into other wiring. We had one guy "I could not convince" yet the answer was staring him in the face----his MSD was causing the voltage regulator to go nuts.

The other problem is some of this aftermarket "junk" may not be very well decoupled (filtered) from the power source. In the case of high- ouput ignition, this might mean that the ignition, if not well filtered ON THE power lead, can send unwanted pulse energy (EMI) right down the power lead to some OTHER component

In the case of your FAST, if THAT is not decoupled correctly, it means that it is more susceptible to EMI from other systems.

A very good example from the old days was plain old ignition interferance in AM or CB or other radios. Some of this came "down the antenna," radiated by parts of the ignition system. But in SOME cases some of it was conducted right on the 12V power lead.

This IS the reason for the capacitor (condenser) in the alternator and on the coil+ terminal in the older cars