There is a guy who runs a rack hanging off the back of the axle on a sky high jacked up 37/39 Chev Coupe here in the Pacific North West. I used to see him at rod trots all the time. I didn't like the looks of it because it looks like a big scab hanging on the axle. I don't know what he ran for a steering column but he did drive the car for years with that setup and may still be running it.
Personally I don't like the looks of it (too cluttered) I am not sure how the rack takes the vibration of riding on the axle instead of the frame and some of the steering column setups to make it work may be suspect as far as safety goes. If it was a good idea to hang a rack on an axle some major rod suspension mfg would have had a kit out long ago.
Advantages may be that you wouldn't have to run a panhard rod and you may solve a clearance problem with that huge engine filling normal usable spaces for a conventional steering box.
If you can stand people pointing it out and questioning your wisdom for hanging it on the axle head on because it will for sure strike up a lot of conversations at rod trots.