Haha, seems like this may be slightly overbuilt then! Oh well, I might take up a new career in stealing lead so it might come in handy!
Just out of interest though, If I was to build a flatbed above the wheel, what would have been the best way to get the height necessary above the axle, without adding too much weight or being unstable? To leave 4 inches of wheel travel, the bed would have to be approx 250mm above the axle (With 13'' wheels, obviously it would be less with 10.'') I'm assuming factory built flat beds have axles which sit higher initially which would help.
most "factory built" flat beds from the likes of ifor williams etc use custom made C channel for the longitudinal chassis legs with C channel or box section cross members for the flat deck floor to rest on. The axles are then bolted to the longitudinal C channel via indespension type axles such as yours or leaf springs as per ifor williams trailers.