while you probably could get it from ford, i wouldn't go through the trouble. the udp means its a smaller belt than stock (i am assuming that the belt is provided with the udp because its a different size, can anyone confirm this?). there are also different belts for ac and no ac.
the easiest thing to do would be to get the size of the belt(its usually written on it) and go to an autoparts store and get that size belt. heck if you're lucky someone on here might have the size on hand. and do yourself a favor, buy a goodyear gatorback belt (i think this is the same that comes in the kit with your udp) they are worth getting.
The third number in the listings is for 6 ribs while the last three numbers refers to length. 817 is 81.7" while the 820 is 82" so the Goodyear belt is .3" shorter which is just over 1/4" shorter. I got these numbers from Esslinger Engineering.