hmmm
I'm not sure I understand you correctly Tony, so I don't dare to disagree;
instead I add it as "OR..."
Brandon, to make things as easy as possible, you might want to take a look at this:
http://www.bmikarts.com/Clutch-Mount-with-Jackshaft-_p_1129.html
I can't tell the JS to PTO distance, but at least what ever it is, it'll be FIXED;
you might give BMI a call and ask for the distance for your engine (model not just hp rating might be important)
then you know if there is a matching belt, and you can simply stick to that.
(assuming you can ask the seller to send your wanted belt size)
Otherwise, you have to do it either way:
A) find a good position for the jackshaft, and see what belt will fit, maybe adjust slightly to have a perfect matching distance (then ask the seller for that size)
or
B) (as I think Tony suggested) just get any belt size the seller has to offer
and work backwards from there;
meaning determine where the jackshaft has to go to match said belt, and find a way to mount it there.
'sid
PS to get an idea what beltsizes are available for series 40 check this link:
https://www.gokartsupply.com/tcbelts.htm#203786A
(you'll get a 7.5" driven most likely)
[
EDIT]
chances are the engine mount is close to the manco part 9334-2
see this manual for a TC breakdown on a manco with 9hp engine and series 40...
http://www.americansportworks.net/pdfs/gokarts-minibikes/model-389/389_operatorsmanual.pdf
then the matching belt would be 203785A (manco 6814)