I have a 1996 f150 that has the manual 4 speed(with over drive) and runs like a scaulded dog; it has the 4.9 6 cyl (300 cu) and from the factory had two cats. The day I picked it up it threw an ENGINE LIGHT which was a cat performance alarm number. Over the past few years it has been alot of trouble getting it to pass because of the O2 sensor fail.
When the previous owner changed the exhaust manifold they used an after market manifold, dualed out and no front catalytic converter. This left no cat between the two upstream O2 sensors and the downstream O2 sensor. I like the way it runs without that front catylitic!
The way I went, the sensors ( they must be good for this to work) remain and the casual observer would see no difference than factory. I found the circuit on line to put in series with the downsteam sensor( google: $4.50 DIY O2 sensor MIL Elliminator). This worked on mine so far and I was able to get the truck inspected.