My 99 E350, they went to mounting the air bag back on the spring. BUT, they mount it forward of the axle, yes, offset. Don't know why.... might get a system for the 99, but I'll see. (The 90 I got off of EvilBay.... cheap.... got a front AND back less then the price of a back.
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Watch your welds- not saying you are a bad welder or anything, but that one bag and the location of the brake line. If it was I, I would get a custom brake line (be it steel or rubber) and mount it on the outside of the mount instead of through/under. If that weld were to ever break and slam into the brake line and smash or cut it....
Wait a minute.... Your truck has stops for a OEM overload spring set up? Were they removed to install the hitch? I find it funny that you have the stops, but not the spring.....