You're expecting to jump 60hp with a new intake, muffs, and cat? That's somewhat ambitious. I would expect you to be in the area of 260hp with those mods. I'd advise you drop the money on PCM, 3" downpipe and full 3" or 2.5" custom exhaust (instead of just muffs). Drop to a colder range plug and pop on a smaller pulley.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne