I have a set of m571 xt cranks that I was going to install on my Chamuco. The middle ring is getting pretty worn and the BB is going to be a 73mm vs the 68mm that the cranks came from. Is it worth buying a new BB and ring for these (if so where is the best online retailer) or should I look at the new external BB cranks? The new LX are online for $161 all over. Are these stiff enough for a clyde at 235? Or should do I need to go XT? Thanks