Do your current fenders come off or are they welded on ? You might consider making them removeable - very handy when loading pallets etc. on an equip. style trailer as well as when opening doors on a trailered vehicle.
My Load Trail equip. trailer has them and are pretty handy. 1 bolt front & rear and they slip up & off the small brackets welded to the frame. Have to disconnect the wiring connector for the clearance light. Would be fairly easy to weld a couple small brackets to your frame and then weld a pair of matching gussets, 1 to the frame and the other to the fender and use a bolt & nut to secure. Those fenders in steel are fairly heavy though but plenty sturdy.