Deciding how to mount axles

I have a 6 inch boxed main frame and the bed frame I anticipate building will lay directly on top of the main frame and be welded, therefore, I do not have enough clearance for the tires without some form of spacing. I can see I need a minimum of 4 more inches. I went to the steel company and picked up two pieces of 2" X 5" rectangle tubing .25 wall. 72 inches long each. (I asked the fella if he could cut them to 72 inches precisely, I got them at 71 15/16 each, heh, not too bad)

During the drive I decided the best thing to do was to weld spring hangers and equalizers to tubing first, progressively that turned into welding a completely separate subframe, cut my channel for crossmembers, weld together subframe.