2.5" exhaust vs 2.25 or even 2"

IIRC there is a line drawing in the '68 Plymouth FSM. I assume Dodge is probably the same.
Poke around the 'net too. There are people who have documented original exhausts.

I mentioned placement because if its really off it can cause bad harmonics. Bad being a relative thing here. The harmonics are just one factor in an exhausts flow and sound. Definately not the dominating one as far mass flow goes. But if your going through this much effort, its something to try to get ballpark correct.