[JesseK]

Sorry Adam, I meant 254/1868. They had a box tube that ran down the side, above the wheel. We like that reinforcement, but don't want to risk warping of that box tubing.

Attached is what I could come up with in an hour. In hindsight, I could still put the sprockets wherever I wanted -- either in the box tubing or next to the wheel -- and maintain the constraints above.

The middle wheel isn't shown because it's dependent upon the attached gearbox. The gearbox would attach using a slightly modified wheel block (though still 2x1). I also didn't add in the 3/16" spacer to drop the center wheel.

Maybe more to come after the dog park.