Exhaust and Muffler Recommendations

How long are the Ultraflows that you used? They sound awesome at idle and on the other sound bite. But how are they on the highway? Any annoying drone? And do they make a quieter version of what you have? What is the typical muffler length for proper fit in a 67 Dart? I'm wondering if stock style "hemi" mufflers will fit.

Karl M.