If you can find a GOOD exhaust shop in your area that would be my first choice. A shop years ago fabricated a Formula S style single for my 66 cuda using larger diameter from the head pipes going into 3 inch all the way back. I used a piece
of 4 inch heavy wall round stainless pipe and "crushed" in in a vice to oval it out
and slash cut the end to replicate the factory look,with a 3 inch Flowmaster 50
muffler, it sounds a performs great with the mild 318 LA roller.
If no luck finding a GOOD shop... you will know if the exhaust guy seems "iffy" when
you explain what you want to do, then move on. next I would get an Accurate Y
pipe which is the same diameter as OEM but improved collector. I have this Y on
another 66, It also works great with a 2.5 all the way out and cheap Summit Turbo
style 2.5 muffler.
Neither of the above is cheap! Probably could put together a dual system for less$$
But if your after the unique look and sound of the original single? For high HP a dual
might be the way to go, mild 318, 273 good, hot 360 nay.
Exhaust manifolds are limiting above maybe 325 HP, if you decide to keep them and go single exhaust take some time and port the crap out them.
Single is also lighter and sound better IMO, no other car made in that era that Im aware of had this type exhaust. Good luck
