I broke one in the middle. I was running slicks and little over 300hp with a 4.11 gear. Don't forget that ratio has a huge effect on how much torque is put on the axle. A 3.00 gears puts 25% less torque on the axle than a 4.11. Do your axles taper in the middle? Some of the earlier, mine were 64 falcon axles, are smaller in the middle. I also had a mII that the axles kept the same diameter from end to end and never broke with slicks, 4.62 gear, 550hp running mid sixes. It blew the caps off instead. The only reason it lasted is because of the weight of the car, another variable in the equation. It is a tough call to say how much it takes to break one. You have to figure traction, ratio, weight, shock to the drivetrain, axle hop, engine torque...... If any of these change, so does the ability of the axles to live. I don't see anything in your sig to suggest you need more than 28 spline though. I think Moser has a chart telling how to figure what size axles you need.