Make sure that there is a vacuum line going to the fuel pressure regulator from the back of the intake. If this is not there, relative fuel pressure will be high at idle and lower rpms causing it to run rich.
If that checks out, get yourself a fuel pressure gauge and make sure that fuel pressure is from 37-40 psi with the engine off. Run the fuel pump and monitor the pressure. You can run the pump by grounding the fuel pump test lead with the ignition in the on position. If you look at the test connector, it's got female plugs on either end and four grouped in a square in the middle. The single connector closest to the group in the middle is the fuel pump test lead. I just made a long wire with a male spade connector on the end that I just stick in there and ground it to the engine when I need to run the pump.
If the pressure doesn't hold steady, replace the FPR.
Considering the condition of the car that you described, I'd check the harmonic balancer and make sure nothing is sticking out. It's possible for the outer ring to slip causing the timing to be off. If you don't see anything obvious, you may want to set your #1 cyl at TDC and confirm that the timing mark matches.
Confirm that the injectors go with the MAF sensor. Maybe the idiot before you tried "boosting performance" by adding 30# injectors. That's way outside of the computer's ability to compensate. Since your O2 sensors were reporting rich and generated a code, it's a good bet that they were working fine. I don't think the computer will report a rich or lean condition unless it can't compensate.
Those are some ideas.
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351W 89 Mustang GT Convertible