There are two different lengths of serpentine belts, depending upon whether or not you have the cold weather package. If you do a search of these forums, you will find lots of conflicting misinformation about how to tell whether or not you have the cold weather package. In my experience; none of the information that you find will be of any use whatsoever. All I can tell you for sure is that it has nothing at all to do with whether or not you have a plug for the block heater. The difference is the size of the pulley on the alternator, but if the alternator has ever been replaced, it might not have the same size pulley as it originally had.
The only solution that makes any sense to me is to buy the serpentine belt locally; that way if it doesn't fit, then you can exchange it for the other size without having to pay the postage.
2001 F350 XLT 4X4 crew cab, dual rear wheels, long bed, black, 7.3l turbo diesel, automatic transmission, 4.10, shift on the fly, running boards, sliding rear window, 38 gal fuel tank, Odyssey extreme 65-PC1750T batteries, Michelin LTX-MS2 rear tires, Sumitomo Encounter front tires, Shell Rotella Ultra elc, 11,500 gvwr, 20,000 gcwr, 7500 gvw, completely stock, 114,000 miles when purchased, now has 250,000 miles, repairs so far: two starters, batteries, cps, brakes, tires, alternator, ball joints, locking hub, wheel hub, AC compressor, serpentine belt, power steering pump, driver's side window, ignition switch, o-ring, Spectra premium radiator, water pump, fan clutch, fan blades, thermostat, thermostat housing, tensioner. Stolen twice so far, but recovered both times with major damage.
2005 Nomad Rampage toyhauler made by Skyline. 11,500lb gvwr.