I'm impressed, you started with a great product. Commercial "service bodies" are built to take a beating and keep on ticking. Way better than anything on the RV market. That should last a lifetime. Any mods you do, try and match that commercial robustness.
Wheel spacers are an option, but price out a new axle built to measure. My 3500# idler axle cost $207CDN, about $145USD. Why bother with spacers if a proper axle is so inexpensive. Any commercial spring and axle truck shop can build exactly what you want. If you do the axle, build it to fit the same rims/tires on your truck. And then you only need to carry 1 spare.