You're talking some bracket fabrication here. Strength in the actual mounting bracket material as well as supporting bracketry. You might be able to locate brkts from a different year-make-model vehicle that you could use instead of a total scratch built system. Lots of pickups use an outboard mounted system that you might use and just swap them side to side and mount to the inside of the frame. Ford Rangers for sure. You'll have to 'box' the inside of the frame to mount them. Go to a wrecking yard and do some looking. Take a tape measure though to get the right width for YOUR springs.