Did you get this car fixed? What was it?
I know this is old, but for future readers I thought I would respond anyway.
Stalling while driving is a very unusual situation. PATS is certainly not allowed to shut off a running car. And the PCM has a backup plan for just about any sensor failing. Since the 99-04 Mustang has 8 separate injectors and coils, any one or two of them acting up would not be enough to stall the car out while driving. About the only thing that could make that car stall while driving would be PCM related problems, or a crank sensor issue, unless the problem is man-made (for example, wiring not where it should be that rubs or burns and causes problems with critical circuits).
Another possible cause of a car stalling out would be that the PCM is shutting itself off. Either on purpose (to protect itself from a short), or an accident (due to a power or ground issue). Or if the PCM itself was going bad, which is a bit rare and least likely.
It is true that P-71 (Police) Crown Vics didn't have PATS, so this car might start with a P-71 computer installed. But the tuning in that computer would not be optimal at all for a Mustang. And they were all automatic cars, so it wouldn't be very useful for a stick shift car. Nor is this necessary these days. If you did install a P-71 PCM in an automatic GT, you would not have to disconnect the cluster or reprogram it. The cluster is the PATS module on your car, but the P-71 computer didn't have PATS enabled, so it really wouldn't matter what the cluster/PATS thinks.
A tuner from SCT, Bama, etc. is just a flash tuning device. By itself, it cannot delete PATS. It would have to have a tuner like myself write a custom tune to it, to purposely disable PATS.
A much more cost effective solution for deleting PATS would be to just send us the PCM, and we could reflash it to delete PATS. Even if the computer is already custom tuned, we could retain that tune and delete PATS on it cost effectively. Usually for less than what a tow would cost, or a key to be programmed.
Best regards,
Steve Hulett
Drag Radial Performance