It appears that the frame is indeed the same on 2500 and 3500. Suspension differences may include the springs and shocks as well...So, it is possible to convert your 2500 to a 3500 but at what cost?
What distinguishes the 2007 Ram 3500 from the 2500 is a firmer suspension and 3,000 more pounds of towing capacity -- up to 16,350 in all. It also has the option of either single or dual rear wheels.
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7.5 3500QC 6.7CTD 68RFE Build date June 2007
Superchips tuner 3808, Boost/EGT Laser Gauge, Airaid cold intake