Resurrecting this thread to report back on the project.

I'm going to say, without any hesitation: DO NOT DO THIS.

The trailers that have square stock frames are not laterally rigid enough to resist torsion forces being transmitted from the longer lever arm of the larger wheels. We loaded the trailer up and got about 10 miles into a test ride when we noticed the whole frame twisting from side-to-side and whipping back and forth between the wheels. There is not enough structural integrity to the wheel mounts to hold them in a consistent plane.

I will grant that we did have a fair amount of weight on there, but I would expect the problem to develop with any amount of weight, given enough time. Initial tests were promising, but the final analysis is: dramatically unsafe.

I would think that some sort of solid axle or maybe a few pieces of rebar welded across the frame would help.

For our upcoming tour, my girlfriend will, sadly, be hauling this instead: