Have you ventured with the search function into the 650B forum? Here's one of many. You can search thru the whole thread aswell. There's a sub-thread link of ljsmiths to more info in one of these posts aswell.there is someone who has "pimped" the epic 26 (year 2011) with fork and wheels 650B (27.5 ") ???
seeing the rear I think there is space enough. maybe whitout mud ...