I believe in replacing O2 sensors since they are the final word on your air/fuel ratio as far as the computer is concerned. If your MAF is off some, the computer will adapt. If your fuel pressure is off some, your computer will adapt. If the o2 sensors of off, the computer cannot adapt since there is no other way for it to determine what the air/fuel ratio is.
As an O2 sensor wears out, it will report on the lean side, which in turn will richen up the air/fuel ratio. Running too rich means fouled plugs. I'm sure his problem has more to do with the MAF sensor being bad, but I still think for a tune-up sake, O2 sensors should be replaced.
Spending $100 every 60,000 miles is a small price to pay for a well running engine.
Jeff, try running a lower octane fuel now that you fixed the MAF. I'm interested in hearing if the pinging goes away. You should also get a timing light so that you can verify the timing.
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351W 89 Mustang GT Convertible