Wow...... 
Well with that much already invested...go ahead and do the O2's. It'll just be adding dirt to a mountain. Plus you can always do what I do....if they dont solve the problem, clean them up with some TB cleaner and return them.. Just be careful pulling the old ones out---they break easily. 
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'91 Mustang GT 5.0 AOD Daily Driver
Current stats: NEW 302 short block, E-303 cam (@.050, 220/220, .498/.498, 110 degrees), 1.6 roller rockers, Performer RPM heads (aluminum, 1.9/1.6, 60cc chambers), Cobra intake, Granatelli 75mm MAF, Billet AFPR, Equal-Length Shorty Headers, Off-Road X Pipe, Flowmasters, 4.10 POSI
13.73 at 103.8, ~2.2 60'
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