Harry (ztuner) says the stock pulley isn't a harmonic balancer. Whether or not that's the case I don't know. However, we are far beyond stock power levels, so I wouldn't want to use a stock pulley anyways.
If the break was occurring further aft on the crank it would make more sense that excessive power caused the break. The crank looks just as weak between 4 and 5 as it does between 2 and 3. Why are we not getting breaks there, where there will be more torque on the crank?