Actually, some ARE studs. They are called drive studs, are the same size as the holes in mag wheels, typically 11/16, and use a flat nut to hold the wheel on Typically used on HUGE horsepower cars.. I'm using 1/2x20 by three and four inch "bolts" on mine.
My comment to Steve has to do with what I have and mine are grade 8 bolts. I know what I have, and I know that Cass stated that's what he uses on these particular axles.