Most of the problem is created by the repositioned sparkplug. With the P heads, the sparkplug is canted into the combustion chamber at a different angle that causes some problems with stock type headers. With some headers, you're fine on six out of eight cylinders but those last two require the use of a shorty spark plug (like Accel) to clear the primary.
Coast High Performance/Pro Mustang were susposed to start making a P specific long tube header. You could contact them to see if they've started shipping yet.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team