The Mac longtubes will work, period. The #7 plug (I think) is snug, but they'll work fine. CCM knows what they are talking about. The webpage in my sig has info on headers and it also says the Mac's will work.
As for the other question, the P's are better. The P heads have better flow, velocity, and the I/E ratio is very good. There are too many reasons to list why they are better.
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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