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I just upgraded my whipple to an 8 rib. I have a new and slightly used Gates K061247 (the part number you need) I'll be selling for a good price. PM if interested.
I think with a 3.125 you could also use a k061223 with the right idler pulleys. You might be able to find a gatorback in close to the same dimentions with some research. Goodyear 6 rib gatorbacks start with 406 and the other numbers are the length. If you could find something between 4061223 to 4061247 it could be made to work by swapping different sized idlers.