The concern I would have about using a poly-v/ automotive style belt for the main spindle belt is that they seem to run *extremely tight* at least in auto accessory drive applications. Much tighter than classic "B" section V-belts, and the cantilevered/overhung load could have a detrimental effect that you might not realize until you're already into it. The upside is there would seem to be plenty of "wrap" on the sheaves.
If you are talking the spindle to feed-box flat belt that's a little different animal. I ordered some of the thin feed-box belts a few years back, don't think I paid that for it, could not tell you where they came from now...