Shawn, I made mine that mounts this exact same way. I found that 3" (if I remember right) iron pipe slid onto the box nicely. I split the pipe on one end, welded on some tabs, then bolted it together to clamp the box.
I dunno for sure, I've never followed the series that much. I know the cage portion has to be made by an "approved vendor", and Hendrix is the only name I've ever heard associated with the F-Toy series.
Same here, have for years with no issues. You have to decide if you would rather replace your wheels, or space them out. The price is almost sixes for a set of spacers compared to a set of cheap wheels.
I've always ran a set on the rear of my GM and Toyota trucks, just to get the track width...