If you are satisfied with the way your car sounds, just leave it. At your power level and intended purpose, you will gain little to nothing with a bigger exhaust. A 2" dual system will support around 300hp. Even if your engine dyno's at more than that, you're not using that HP just cruising around, and the smaller diameter pipe keeps exhaust gas velocity up which helps with low end torque. Now if you're making a lot more power than that and you're racing, everything I stated above goes out the window....
To me, a cruiser doesn't need headers. If it has headers on the car when it is bought, that's different. If the owner is installing headers, that means he is after a certain level of performance, such that he is willing to put up with the downsides of having headers. In that case, like I said, I would use a minimum of 2 1/2.
I think as i said before drive until he needs three inch. After the sun comes back out and he drives it he may want to up to a little more power. You and i both know its hard to leave a mopar alone. So for now i agree leave it alone.
