It sounds like an expensive solution to change both pulleys to a poly-vee belt configuration... if the tensioner is not proving to be effective, it may be because the belt is old and may have stretched a little, exceeding the limit of the adjuster... if that is not the case, then replacing the locking washers associated with the adjuster bold is a must. I have seen the brackets so badly 'mashed' that even new locking washers are not effective - if this is the case, then replace the adjusting arm too... still much cheaper than new pulleys!
regards,
Bertybuoy