273 Y-pipe?

If originality isn't a concern, but you still want the cool HP single exhaust, here is an idea. These flow REALLY well. You can just have an exhaust shop use one of these and make a Y pipe. I plan on using the dual 2.5" into one 3" on my 63 Dart.

http://www.flowmastermufflers.com/?page_id=11248&sub2=Scavenger YCollector