Dave is right on. The Flowtech's have been rumored to maybe fit, but I haven't seen it. Check out the website in my sig, it has all sorts of header info on it. There are a few different headers that'll work with the P's in a Fox stang. Ford and Mac both make short headers that work. The regular Mac longtubes will work also. I am probably going to switch to the Mac longtubes, so my Ford P shorties and off-road H pipe will be for sale soon. If you are set on keeping the Flowtech's and if they don't fit the P's, you may just want to pick up some used GT-40 irons. You can find them for around $350 - $500 and your headers will work with them no prob.
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1991 Mustang GT - Cobra Intake, FMS 65mm TB, C&L 73mm MAF, GT-40P heads, Crane 2031 cam, Crane 1.7rr, Built AOD, Art Carr 2400 non-lockup converter, Transgo shift kit, BBK chrome cold air, FMS 'P' headers, Pacesetter off-road H, BBK aluminum pulleys, 2-C Flows w/ dumps, 4.10's, Steeda STB, for now...
The Original GT-40P Tech Page