OP—just curious
What engine, transmission, axle ratio, & wheel size are on you present towing vehicle? Also, I apologize for myself and the others who might seem to be stomping on your dream. In the end it is absolutely your choice, we’re just trying to share.
Just as guess—the 350 cargo rating is lower because there is more spring hardware. Also, options will knock off capacity. Out truck dropped 400 lbs. from the spec sheet to the door sticker.
I forgot in my post that your cargo capacity will be 724 lbs. higher because the gas engine weighs that much less than a diesel. However, that additional weight comes mostly off the front axle. The 2013 configuration says 623 lbs. off the front and 101 lbs. off the back. So, with more or less the same rear spring in every configuration of a 250 the backend will still be drooping when you get over 2000 lbs. of pin weight on your future trailer. Another way to get some insurance on rear droop would be to order the camper package on the 250 which will get you the aux springs. Don’t know about the spacer block size with a 250 camper package. You could always add air bags to the rear in the future if you get too much droop.