P heads were on the v8 explorer/mountanier and were actually supposed to be cleaner then the reg gt40, were on the 96-02 in fact when most cars had switched to a 4.6 due to emissions. the gt40p should be no problem to get to pass smog in any of the 50 states since they were on 50 state legal autos. I would however not get them if they have been ported as that can change the smog outcome.
Well they dont have the thermactor passages drilled in them. But i'd suspect you coudl still bolt the system up to make it look operational to pass a visual inspection. From what i've heard the system only pumps air to the heads on cold startup and then to the cats once the car is warmed up. So just making sure your car is warm should suffice to have it pass easily.
and as blksn said, these heads are more efficient so they should have no trouble passing outside of that little prob with the thermactor. I'm still waiting on confirmation of what i've heard as i am not 100 percent sure my explanation of the thermactor system is completely true.