There have been several FFR guys with 4 into 4 headers that have put the O2 sensors in only one pipe on each side and had no problems. I also asked the question on
The Corral , and it's pretty standard procedure for running long tube headers on Mustangs. As long as everything is working right, the A/F ratio should be very close to the same on all cylinders. If you've got a bad injector or something on one of the cylinders that has O2 sensor, it would give a false reading to the computer. But if that's the case, you've got other problems.
Pete