[Ram3500Dually]

You need a 4' ID at one end and a 4" OD at the other. Here is a thread that has pictures and part numbers for deleting a muffler or a cat.
http://www.dodgetalk.com/forums/showthread.php?t=56837
I can't remember if the 03 has the offset muffler or not like the 05

 

[Ram3500Dually]

I posted the link above with all the information including part numbers and pictures.