The best thing to do as a preventive measure is send one of the gearboxes to Bernie and have it shot-peened and polished. Will stress-relieve the gears and give it a bit more added strength, you may then get away with this for a period of time, providing you're not some animal with the 'box / clutch, as it is the shock loads that kill the MT75 (especially when the torque comes in with a big BANG with a T3/4). You could always fit a T4 and that will further reduce the stress on the transmission by moving the peak torque point higher up the rev range, meaning that you are travelling faster when it comes in, so less likely to break the 'box.