BBK longtubes barely clear P heads, not sure about the shorties. GT40s came on early 5.0 explorers and Cobras, if I was going to do it again I'd just hold out for a set of the regular GT40s to avoid any issues. As far as performance, you'll hear from some that the P heads are better but honestly they are pretty much identical as far as performance.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.