Need second opinion about possible bad ECU.
 

So I've recently bought a 91 TII s5. It was originally an NA, and the last owner swapped the engine with a JDM engine.
When I got it, it had the usual problem of not firing the rear injectors with the n374, so I'm currently running the n350 from my NA in it, and it runs great. I'm just making sure that I'm gentle with it.

I purchased a N370 Ecu from ebay, and after receiving it, I noticed that part of the case is a bit rusty. But I went ahead and plugged it in, and here's my problem:

Car won't start with it. The engine is turning over, and I smell gas, but that's as far as it goes. I've cycled through all 3 ECUs, and the 370 is the only one that won't start the car.

I contacted the seller and he assured me that it was tested in an rx7 before he sold it. So before I go escalating this case, I would like to know if anyone has any idea if there would be another explanation for why just that ecu won't start the car.
To me, all signs point to bad ecu.