Run a string around the pullies, apply full tension to the tensioner and measure the string. Very simple and works every time. I've done this 4 times already.
Then go to this chart.... find the number of ribs on your belt (usually 8) go to the length you just measured. Take the number on the left column and put it in the search box on Rockauto.com.
I know for me there had to be someone out there with the same combo. 2.3 whipple w/3.25, stock lower, and 4 BF idlers. And it wasn't a matter of being lazy, just not having thought about the yarn/string technique. But once enlightened, it made sense and worked.