Sorry but Ram still uses 8x6.5" bolt pattern. 2013 is the year they added 18's to the HD lineup and they fit just fine on older trucks as long as the truck hasn't been converted to manual hubs. While most of the wheels have changed from 2010 to 2014 there are a couple DRW wheels that carry the same part number between those years.
To the OP you need to get to a scale. The 18's from a 13+ will only get you up to 7K with the wheel rating, thou the tire rating may be higher. Your 03 wheels and tires will get you to 6390lbs.