The Mac and Ford shorty headers that are P specific are both 1 5/8 headers just like any other aftermarket header. I'm not sure about your H-pipe, but if it is a ball and socket type, then you can use any longtube header with it. Again, if you are set on using your exhaust setup, you may want to consider some used GT-40 irons or the Windsor Jr's. Of course, the Windsor Jr's require stud mount roller rockers, so there is extra cost associated with them. You will probably need hardened pushrods with them also. The GT-40 irons will accept everything you have now, no changes needed except a different spark plug I think. Even if you did decide to go with the Windsor's or the Edelbrocks, they are at least $500 more than the P's and a new exhaust setup (new headers and H pipe) is only going to set you back around $350 or so. Plus, you can sell your Flowtech stuff afterwards. Another side note is that the only long tube that will work with the P's is the Mac longtubes, who is also the only company besides Flowtech that makes a longtube header for AOD stangs.
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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