My choice would be an extended cab short box. Ford offers the combination, it is only a bit more than six inches longer than a regular cab SD. I would want the rear seat space for luggage and other gear. A door opens into the rear space on the SD Supercab.
Crew cab is a foot longer for either bed size, most of that is added wheelbase.
The rear seats fold up, but I would be tempted to remove them if expecting to never carry more than one passenger. Just as I removed the rear seat in my 12 passenger E-350, planning to never carry more than eight people.
Lower GVWR options are to work within tax and licensing laws in various jurisdictions. There are as many as 15 GVWR ratings for the F-350 SRW, which ones are available depend on model, chassis options, wheel and tire sizes.
If always towing, I would choose the 3.55 axle ratio for the diesel, to let the transmission spend more time in higher gears, although there is not really that much difference between 3.31 and 3.55.
Up to 10,000 GVWR you can get nearly the same rating in the F-250. In both models, the 4x4 gets the heavier GAWRs, and since you are buying the diesel, which adds about 800 pounds to base empty weight, you might want to go for the higher ratings to make up for capacity you lose.
