The original style 1/2 ton axle won't swap directly in. 1 thing would be the torque tube, which isn't long enough for the 3/4 ton chassis, the other would be the spring mounts, namely that fact that the 1/2 ton uses a pivoting mount and also mounts above the spring, rather than below it like the 3/4 ton.
What I'd suggest, since you seem to be going for a hot rod, would be to use the drop axle as you mention, with a disk brake conversion, then use something like a later model Camaro ar Trans-Am rear axle. That will convert everything to the same bolt pattern (5 x 4 3/4) and get you an upgrade on the brakes at the same time. This will, of course, reduce the load capacity greatly.
