Well...if you're doing the 408 I would do the 3". I have 3" on my 69 B'cuda....I have no idea why some of these guys say it will be so much louder than the 2.5"?? Use some good pieces like TTI,absolutely use tailpipes(no turn downs)an X-pipe and pick your mufflers wisely(no Flowmaster 40's)Mine is more than reasonably quiet(meaning you can talk without yelling) at highway speeds but it has a nice growl when you mat it!!