Runs too rich and very poorly?

Hello, you're limited on readable data for a 1991, what is the part number for the working ECM? We can look it up and see what the differences are. I assume it's from a 91' as well. The wiring diagrams from that far back are very basic, but
You can start by checking some sensor readings if you have a multimeter, make sure that the Oxygen sensor voltage is not stuck lean, (low voltage), that will send the ECM into a rich strategy. The Oxygen sensor looks to be a 1 wire sensor, so with the vehicle up to operating temperature, take a voltage reading while its plugged in.
Does the Instrument Cluster have a temperature gauge? The Coolant temperature sensor will send the engine rich, vacuum leaks, bad Cat. Are you getting a lot of smoke out the back, how did you determine the rich running condition?
The MAP sensor voltage at idle should be in the lower range, around 2 volts, which will be with high manifold vacuum, as you open the throttle manifold vacuum will drop so the MAP sensor voltage should go up, if you suspect a clogged Catalytic Converter, you can do a back pressure test.

The wiring diagrams only show a one wire Oxygen sensor, but they can be incorrect sometimes, if there are more than 1 or 2 wires, then it is a heated oxygen sensor, and you'll want to make sure that the heater circuit is functioning, or the sensor will be very slow to respond. So, check the exhaust manifolds for any sensors, if none they check farther down the exhaust. We can have you test it right at its connector to begin with. I will post what there is for wiring diagrams and you can let me know if they look correct, GM kept everything pretty much the same back then.

It looks like the MAP voltage at Key On engine off should be more like 3.5 volts at sea level. This old of a vehicle will have an analog MAP sensor it looks like.
Smoke testing the Intake manifold will reveal any vacuum leaking areas, I'm sure this truck will have multiple vacuum leaks. Lines start to fall apart when they age. You can also hook a vacuum gauge up to the intake manifold and see what the vacuum is at idle.
If you're planning on junking the ECM with no injector pulse, I'll take a look at it and see if its repairable. Just something I do on the side, if I can fix it I'll send it back to you.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-test-an-oxygen-sensor-02-sensor

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

It depends, are you finding wire that is cracked and the insulation is missing? With a vehicle this old there can be wiring issues all over the place. The flashing code 12 means it's in diagnostic mode. It has been so long since I've been dealing with obd1 systems. If you disconnected the IAC and it ran better, and you're hearing a pulsing noise from the valve, it's most likely failed, and the ECM might be trying to move the valve and it can't. Or the cracked wires are corroded inside and there is voltage drop on the IAC circuit.
Also, if the last ECM didn't have any injector pulse, a couple possibilities are one of the injectors is shorted out internally and it took out the ECM driver for the injectors. You may want to check the resistance of each injector. When I see an issue like that it's almost always a shorted component that takes out the computer.
You know it's a fueling issue if it will run off propane or whatever you're using. So, it could be a fuel pressure issue, its most likely not the ECM if it runs off an alternate fuel. That means you're not losing spark.
Are you just not seeing any more injector spray into the throttle body when the fault occurs or are you actually checking the injector pulse with a test light?