2.5 inch exhaust is a really good size to use when you are going to be using dual pipes (as with headers like you are). With a healthy 400 you COULD use a 3 inch setup but with 3 inch dual pipes you might lose a little backpressure and hurt the performance during your powerband (only by a couple HP I'd imagine).
I went from 2.25 exhaust to 2.5 on a healthy 350 and really loved it but im not sure i'd go any larger.
If you want the look of the bigger exhaust just get some flared exhaust tips that are a 3 inch design (these actually look really good in the right applications).
In any case its an awesome car to work with and a nice engine as well. Have fun with it. Im sure someone will have more info on exhaust shortly.
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