The factory 69 400 RA III exhaust is 2 1/4". I've got 2 1/4" pipes with long branch manifolds, two resonators and a transverse muffler which is pretty much the RA III exhaust and it seems like it breaths plenty well for my 389. Unless you're really hardcore about making some serious horsepower, 2" or 2 1/4" pipes are plenty. But that's only one man's opinion.
