The brakes are not the same as smaller trucks. the 6400 has not much in the way of compatibility with the 1/2, 3/4 and 1 ton's. Another resource for you would be
www.stovebolt.com, They are into the larger trucks and many guys there have the 6400. I don't believe a 1/2 ton axle will even be close to bolting up. Your looking a larger truck axle swap for disc brakes