The Cobra axles are shorter, but there is play in the inner joints that will allow them to work. I am quite sure that the torque capacity is reduced due to the loading of the inner joint because of this.
Shane, you have much bigger things to worry about that breaking axles. If Gary can launch his 3800lb SC at 4000rpm on ET Streets darn near pulling the wheels off the ground, (oh and did I mention 480rwtq) without breaking anything, you will be fine with road race tires. I don't recall if Ted Lyons has had any axle shaft issues but he also has over 600hp.