P headers just have the tubes moved further out from the spark plugs to give them more clearence. The P heads have the spark plug at a different angle.
P headers will work on stock heads, but stock type headers will not work with P heads.
89sleeper summed it up nicely. I'd add - there appear to be a handful of regular headers that will work with the P's - but it's not by design; it's because aftermarket header manufacture is in general a VERY low quality process. Sometimes a header comes out that has it's primaries set in a way where there's JUST BARELY enough clearance to get the plug wrench on even with the P's. I don't think you can count on the fact that because someone posts and says that "X" brand header worked with his P's, that that will occur everytime with that header and your P heads.
Also - folks have posted that there are shorter plugs available that solve the plug clearance problem -- but I don't have any firsthand knowledge of that, much less a brand/part number.
88stangman - GT40 irons, and GT40P's are two different heads.
GT40 Irons - available originally on 93-95 Cobras, and 95 through the first two production months of 97 Explorer/Mountaineers
GT40P's - originally available on the last 10 production months of 97 - 2000 Explorer/Mountaineers
GT40Y - called 'turbo swirl'; aluminum heads from FRPP
GT40X - called "Xtra performance"; aluminum heads from FRPP
GT40Z - latest, highest performing aluminum head from FRPP
No wonder when someone says "I've got GT40's..." it's confusing.
I've also heard of the shorty style plugs working, but i've also heard they are like $25+ a plug, so for 200 bucks you might as well just buy the headers.