5.0 Weird Tech Issue

OK folks, I have a rather strange problem that I could use some advise on.
Background:
Just purchased a '33 Willys Coupe with a 5.0 5 speed allegedly sourced from an '89 Mustang GT. It runs and drives but was reported as being "down on power" by the previous owner. The PO inherited the car from his deceased father and knows little of its history. It seems to start and idle OK but did fail to start once after warming to operating condition. Transport driver gave it a shot of starting fluid which caused it to catch and it ran fine. It appears to have a stock 5.0 engine with the exception of a Cartech upper intake, and the elimination of all emission equipment. I put an OBD1 code reader on it this afternoon and it logged the following codes: 21,31,81,82,85 and 84 engine off. Engine running it gave 6 (isn't this supposed to be number the of cylinders???), 21, 41,31. So I am attributing everything except codes 21 and 41 to the removed emission equipment. I made sure the engine got up to temp and the code 21 remained. I measured the ECT resistance at 1.5 Maga Ohms.
Thoughts on what I have? Did someone replace a failed ECU with a 6 cylinder one? Would it even run? Oh one more weird thing, the code reader didn't go into stored code mode after the engine off test, just repeated the original codes and then stopped. Never got a code 10 indicating a switch.
Any guidance greatly appreciated.
 

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Some questions come.to mind,
What computer is installed, was it sitting for a long time, in tank or external fuel pump?
can it pull wheelies?
Not sure on the computer. I think it's under the passenger seat but haven't gotten that far yet. I read that it's not unheard of to have an A9L v8 computer to throw a "6" at the beginning of the KOER test (hope that's true).
The pump is in the tank. Previous owner said he changed the pump and cleaned the injectors. Believe it is running stock injectors and pump. Fuel pressure check in the near future but would think it would also throw a 91 if that's the problem.
Wheelies? Not at the moment. Maybe, once I get the issues sorted....
A couple of shots of the engine. No air pump, no EGR no cylinder bank bypass tube....
 

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Just replaced my ECT and code 21 is resolved. Now onto 41. I also ran the "compare test which flagged 80, 40 and 20. If they were all on the same bank I would understand, but they're not... Think I will swap the Oxygen sensors next and see if the code stays with the bank or the sensor. Also run a fuel pressure test. Not too far from resolution I think.
The ECM is obviously testing 8 cylinders so the code 6 was obviously a red herring. Oh, and the thermostat was installed backwards. That wasn't helping anything...