OAD, depending on what bronze you use you might find it can't take impact forces..I made bushings out of oillite bronze for a machine and they fractured under the stress impact..I personally think if you use steel with zerks and pump some grease into it every time(or nearly every time) you use it you will be fine..go as big as you can on the diameter of your pins for the most working surface to spread to load onto..as far as your tubing for the hoe I think I would keep with 1/4 or better on the wall thickness of your tubing..when you can go rectangular.You can find deals on eBay for your cold-rolled pins and DOM/mechanical tubing,lot of people selling drops for cheap.