XP is the largest cam you can run without having the lifter guides machined. Plus it gives a nice lumpy idle. Everything to do the the cam install was $660 shipped. This included the cam, 105s, modded retainers, valve seals, install kit with cam/crank bolts and valve covers, and a new timing chain. Rated hp is 40-55 range.
The S1X cam sold by Intense is very similar, though just a hair smaller (you can compare the specs). Either would be great for ultimate power on stock heads.
Honestly, if you want to go with reliability, just get a VS cam. You'll still get benefits higher than the rockers (for cheaper) without the timing component wear associated with the larger cams/rockers. You'll also get your mileage.
If I'd went VS cam, the whole setup listed above would have cost me about $200 less, because you don't need the springs, retainers, or valve seals. Highly suggested imo. If my GP was going to be a dedicated DD, it would have gotten the smaller cam.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne