only belt ive seen break was because a rock got in there. id say check to make sure the pulleys are lined up correctly before making the switch to chain.
What h.p. have you at the rear wheel?Did those belt brake after the tire was changed?I beat the hell out my softail even pot a wheelly with my girlfriend on the back and never broke a belt.That was a 2000 fxst.I saw my friend broke the belt,a wide one on is ultra,theday after is tire was changed.Probably the beltadjustement was too tight.