I can't remember the part number and have the belt sleeve packed since we just moved. All I did was to take the old belt down to the auto parts store and have them measure it and asked for a 1/2" shorter belt and that did the trick.
I took a look last week for someone and found that the lettering on the belt had worn off. What Dan-d said is the way to go, and is just what I did-take the belt to an auto parts store and get the next smallest one you can find. Someone also said that they used the same JR belt when they put the 2.6" pulley on and got just a little bit of slipping. Another option-call JR (Moss Motors) and ask them what size the "stock" JR belt is, then take that number to a real auto parts store and have them look up the next smallest size belt. Good luck with your search. BTW, what are you going to do for fuel delivery/management?