You need O2 sensors so that your computer can deliver the correct amount of fuel when cruising and at WOT. Without them, your engine will run rich all of the time and foul your plugs. If you are able to adjust the fuel pressure to lean out the cruise mixture to the point that you don't get fouled plugs then it is a good bet that the fuel supplied at WOT will also be too lean and create excessive heat and possibly damage your pistons. I'd got with Rebel79's fix.
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351W 89 Mustang GT Convertible