I realize he doesn't need an alternator, cadunkle. He needs an alternator belt, and depending on the alternator size, his application uses one of two different belts: one fits a 60 amp alternator, the other fits a 70 amp alternator. I couldn't find any stamp on the case of my alternator when I replaced mine a year ago, so I just took the alternator with me and the 60 amp version is what mine turned out to be.

Since you don't know what size your alternator is, you will still have to look at both available alternator belts to see which one fits. I don't think the belts have a measurement size on the packaging. Sometimes the belt sizes can be pretty close, and it is hard to tell what size is actually needed just by looking at them. That is why I suggested taking the alternator with you so that you may first find out what size alternator you have, then you will know for sure what the correct belt is.