Jeff is right on. They will not fit because the spark plug is rotated in and up (towards center of combustion chamber). This places the spark plug very close to the header primaries. Most headers start the downward turn too soon and the result is burnt plug wires. The P specific headers extend from the head a couple of inches more before turning down. This allows plenty of clearance for the plug wires.
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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