I signed a form just like Dave did, but my axle ended up just like yours with about 1/4" between trailing arms and the frame if it is 60" wide. There is only 60 1/2" between the trailing arms when they told me it would be more. My hub face width is 72" as they stated it would be. And, my tires stick out a little more than I wanted, but I am making my own fenders, so I can compensate for that.
Rather than go back to the dealer and wait another 3 weeks for another one, I just cut my chassis steel width 1/2" shorter than planned (59 1/2" rather than 60"). Now, when my 1/4" or 5mm luan is extended down the side of the chassis frame, I'll still have 1/4" between the trailing arms and the plywood. If you can't alter your frame, making a cutout and putting another piece of plywood inside to cover it would work. The cutout would be rather small, probably less than 1/2 square foot.