I did this exact thing on my '91 truck, so I know what problems you are facing.
First of all, the transmission is not electronically controlled, so the only thing the computer is used for is torque converter lockup. You will need to buy a lock up kit to control this.
The ECM is also not used for any of the gauges in your truck. They will all work perfectly provided they work now and you have all of the sensors (oil, water, fuel) installed. The idiot lights also work, except for the check engine light (obviously).
I removed the whole harness and basically disassembled it and removed the wires that I did not need (injectors, TPS, MAP, etc.). It's not very difficult to do this right, just start at the inside and remove the ECM, and untape the harness so you can see that nothing is holding it in the truck anymore. Then unscrew the grommet that holds the hole thing in the vehicle from under the hood and pull it out. I used wire diagrams that listed what each wire did to decipher what I still needed, and in the end, it wasn't much. In the end, both connectors that went to the ECM were no longer needed.
You're problem's sound like they may be ground related, and this will not fix those problems. Get some wire diagrams and start tracing down the grounds. I honestly wouldn't waste the time or money in a new harness.
What specifically is going on with your electrical problems? What happens with the blinkers, fuel system, etc?
TBI is nice if it's your daily driver and having a carb on a TBI vehicle is illegal. I tried tuning my TBI for years and spent thousands of bucks trying to make it run good. I finally threw in the towel when I couldn't tune it enough to make it pass emissions. I installed a carb setup and it now passes emissions with flying colors and also hauls ***.