Your 3500 SRW has the same engine tranny as the DRW so a bigger truck or more GVWR won't help with the burned clutches.
The Cummins has plenty of go power its the auto tranny that needs assisting regardless which year model truck.
Looking at Dodge body builder guide for '07 show the SRW has a higher tow rating than the heavier DRW.
I would put some bucks in the 68RFE for a good "built" tranny like a Goerend or a Suncoast/etc or if you trust your tranny guy go with him. These trannies are about as bullet proof behind the Cummins as you can get.
