After 4 years of running like a champ, my 440 PowerWagon mud bogging truck decided to give me a giant backfire through the carb, enough to explode the air cleaner. Rev's to the moon in neutral but pops out the carb and stalls with any load.

I have changed plugs, wires, coil, distributor, ecu, ballast, ignition wiring loom, cap and rotor. Changed out the 750 Holley to a Thermoquad then an edelbrock 750.

I noticed during all this my autometer 5" in dash tach started acting weird when cranking bouncing everywhere. A friend suggested I unhook it, and it seemed to "fix" the issue although part of me thinks its just waiting to mess with me again and that that a tach wouldn't cause this.

Yes, they have been known to cause the problem you are describing.

The tach connects to the coil and will ground out the coil if it goes bad. Because of the light wire to the tach its not a good ground so the car tries to run. Check all the wires around the coil, dizzy, and control box. Disconnect the tach and go mudding. Best bet is to put on a MSD 6AL, the tach connects to that and will not ground out the coil, but, I would not hook up your tach to anything except maybe a ford till I was sure it wouldn't cause the problem again. It just maybe the wire from the coil to the tach is shorting out somewhere..

Internally...
A tach like any other electronic device can go bad...
If it develops a short..
It can act strange and may cause the ignition system to fail.

Just my $0.02...

If it runs well with the tach disconnected I think you have found your problem. Hook it back up, if symptoms retrun, disconnect again REPEAT until you are comfortable with the diagnosis, Don't OVER THINK IT
MY

Yes! I chased a problem in my hemi cuda for a month. It would work sometimes and other times it acted as if it was dumping the floats and flooding the engine. Happened to see the tach jump one day and disconnected it problem gone. Turns out it was bad and screwing up the msd 6al-2signal. Replaced tach and all problems are gone.

Would it help to put a diode in the tach signal line to eliminate a potential shorting out?

I had an Auto Meter tach go bad in the late 70's. Engine was missing, popping and the tach needle was jumping all over the place. I thought the jumping needle was being caused by an electrical problem in the ignition circuit and started changing parts. After replacing everything I could I disconnected the tach and the problem was cured.