2 - 2.25" pipe with "H" or "X" equalizer should be fine. If you run your pipes back between the rear frame rails and gas tank, then 2.5" is maximum. If you are planning on a stroker motor or 351 swap later, you might want to go a head and run 2.5" pipe now. Otherwise, I wouldn't bother. Remember that you exhaust will flow only as good as the most restrictive section of your system - that is usually your mufflers.