Tom Hunt of www.modelairtech.com
has a lot of experience with large belt drives. I used a couple of his for a few years. Unfortunately, I don't see them on his web site anymore (guess outrunners did them in). But you could talk to him, he is a very helpful and a great guy.
If you look inside the clear cowls on P109 and P131 at this link you will see examples of his belt drives:
Hope this helps,