[AustinH]

So, some interesting things happen with 3" tube and a 50t-72t spacing.
http://i.imgur.com/pE2oye8.jpg?1

Lots more structural webbing is available even if you open up slots large enough for installation of the 72t gears through the top. A 50t gear on a 1/8in drop center pattern has a minimum of ~0.015 clearance to the inside wall of the tube. Unfortunately, since McMaster Carr lists a wall thickness tolerance of +/- 0.019", odds are at some point it'll interfere. I may try a 44t gear next (though this will shorten the wheelbase even further). The other option would possibly be to eliminate the drop center and add omnis to the front and back and sizing the wheels down or up to either 4" or 6".