Interesting. I remember considering that problem, but it got lost in just getting back on the road.
The problem with this adapter is the loss of 1 1/2" shock compression, unless you have lots of lift. If you made the bottom shock mount wider 3", then use rectangle tube (about 2"x3"x2"h) to fit inside the new shock mount with bolts going toward outside (use shoulder on bolts to keep from making it tight) to bottom mount. Then on same plane on other sides of box drill holes for the shock bolt to go through. Shock mount would be in the middle of box. You would get the same swivel shock mount without the 1 1/2" loss.
Just a thought. Hard to explain.