If the breaker bar doesnt fit....then try something else, or shim it into place. You do not take the tensioner off the mount, just to change the belt, and if you have to this....there is something seriously wrong with the tensioner itself.
Are the bearings in the tensioner ok? Has the belt warn normally, or is there something else wrong here. From what you have posted, it sounds like there is something else at fault here..
If you need any more help, dont be afraid to ask......
