backfire = bad O2 sensors? long

[89GTCOUPE]

my 89 5.0 has ben backfiring once or twice under heavy-load wot, but after I cleaned and regapped my plugs, It's been doing it all the time; even under part throttle. the plugs are at .055,timing at 12BTDC, the cap and rotor are fine, the wires have low resistance, and are not crossing. The MIL is turning on after 5 or 10 minutes whenever I drive it, giving codes 11 then 33,41,and 91. my air pump is taken off,so the egr code doesnt bother me, but I just replaced the O2 sensors 2 years ago. Could they be bad already, and causing backfire?
My engine is: rebuilt stock short block, TFS street heads and intake,F303, 75mm MAF, 65 TB, FMS30# injectors(too big, I know),crane FPR set at 32psi(as low as it goes), etc..
Any and all opinions appreciated.