you are simply paying for the higher payload sticker going with the 350.
also take this into consideration, you get the HCPP for free by going with the 7.3L gas motor over the diesel. Not only is there the "diesel premium" but if you order the diesel you need to also order the HCPP as well, if you order the 7.3: is comes standard as we know all 250 and 350 Tremors are identical, suspension wise.