Valves are 1.84" intake and 1.465" exhaust. They do require a P-specific header to do things right mainly due to the repositioned spark plug compared to other production Windsor heads. Non-P-specific headers have been known to work, but require either a shortened spark plug, dimpling a header tube, or making a special spark plug wrench. Using a non-P-specific header is almost guaranteed to melt plug boots on a regular basis.
One of the guys around here (Skankin) has a good page dedicated to the P heads but I don't have the URL handy. Check some of the other head related posts going in this forum.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team