well my uncle just bought an 06 cummins and he was askin me to search around for the best exhaust for his model and other modifications for him to tow better

 

Its already got a full four inch system.Just do a muffler swap only.Leave the cat and rest etc.Lots of MUFFLER choices do a search on mufflers and inquire with some of the vendors who advertise here.Muffler choice is LIKE oil choice LOL.

 

Just start towing. If he is having trouble, either there is something wrong with the truck or he needs to step up to a medium duty with a larger engine.

 

5" exhaust is waste of money, tell him to leave it alone and "Drive it Like He Stole it"
If he wants mods add an exhaust brake, put a 5th wheel hitch in the bed and do some HAULING