exhaust 63 Dart

I assume you have a \6. If so 2 1/4" is the way to go all the way to the back, use a muffler that gives it some back pressure otherwise it will be a dog off the line.

This improves the mid and upper range and wakes up the \6.

The factory exhaust manifold has a 2 1/4" outlet, the head pipe is 1 3/4", the stock muffler
has a 1 3/4" inlet and 1 1/2" outlet needless to say the stock system is very restrictive.