Position the gearbox between the center and back wheels, lightly fastening it with a 5/8" 1/4-20 socket head screws, 1/4" washers, and nylock nuts. The gearbox should be loose enough that it can still slide along its slots.
Cut chains going from the gearbox to the back and center wheels, and attach them over the sprockets.
Cut the 2 cross hex extrusions to 16 7/8" in length.
Fasten the cross hex extrusions between the gearboxes using 1/4-20 thread rolling screws. Drive them in using a 3/8" wrench or socket driver.
Slide the gearboxes into a position where the chain tension is about equal and fully fasten down the gearboxes by tightening the screws and nuts.