well, do you plan on using lockers? if you have open diffs, or maybe a limited slip you will have axle shafts last longer. i have a buddy that runs 40s on his k5 that has the 30 spline shafts with an lockers in the front and a heavy right foot and completely blew up his front axle. he now runs an open diff and hasnt had any problems so far. he just replaces ball joints and stuff like that alot.