2" would be fine, and no bigger than 2 1/4 if you ever plan to hop it up a little during a rebuild. Most people notch the passenger side of the torsion bar crossmember and transmission mount to keep the pipes tucked up neatly on both sides. Be sure to get the muffler shop that makes your new system to use aluminized pipe. See if they will let you brush the welds and treat them with Eastwoods manifold coating while it's all fresh and up on the rack.