[argonaut]

The axle in the photo looks to be a semi-floater to me. Not sure if that matters to you or not. Springs or no, I'm betting the spring-to-spring width on the newer axle is different from the old on, but i could be wrong.

You can easily adapt disk brakes to the HO axle http://www.ruffstuffspecialties.com/...g/EHO5272.html
But the trick is getting a small bore master cylinder for the rear. These brackets fit GM "D52" type calipers. but you'll need a small bore, smaller than the fronts. 78-79 Eldorado calipers and wilwood calipers will work if i remember correctly.

The newer axle is probably a safer route if you don't mind the fab work and welding.