Use the original axle and spindle, and bolt the dirt track load tube over the spindle, using the backing plate bolt holes. Cut the original spindle back a little if necessary. That's how we did it on the 55-57 Chevy spindles everybody ran back then. You might have to fabricate caliper mounts to fit whatever disc and caliper assembly you choose to use. All this is basic "Dirt Track 101" redneck engineering.
When I'm fabricating a brake system from scratch, I get the spindle and hub mated properly, decide on a disc to use, and then use air pressure to hold the caliper and pads clamped tightly onto the disc. Then it's simply a matter of fabricating suitable caliper brackets to attach to the spindle to hold the caliper in the correct relationship with the disc. Make a template of thin plywood or even layers of cardboard glued together to fine-tune the shape of the caliper brackets before you start whittling on steel!
Jerry
