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I would imagine FM makes their exhaust in both sizes so you'd have to measure or compare it to the mid pipe size to be sure. 3in is considerably bigger than 2.5in so it shouldn't be hard to compare the size of the mid pipes to the catback pipes.

You can run any size header you want. The most popular size is the 1 7/8 since it fits perfect and flows extremely well. If you go by Jannetty's header test, each size increase showed a slight increase in peak HP/tq while not suffering down low. Many, of the 1 7/8in headers have a 3in collector and use the off road pipe or HF cat to reduce from 3-2.5 for the OEM size exhaust. 2in headers are typically run by guys that have aspirations of making huge power. From what i've read, they fit but are a bit tight.