A friend just built a 29+ and went with the panracer Fat-B-Nimble 29+ tire, We both have the same 50mm rims and my Bontrager Chupacabras were noticeably wider than his panracer...... they actually measure just a hair over 2.75"
when i measure. the 3 areas where a big tire will rub are on the 2 frame rear triangle spots and where tires passes the fork of those 3 locations, ..........the smallest measurement is 3" and that is the top of rear triangle -frame section