[AndyB]

Do you already have your frame made? It looks like it could be a little flimsy and I would recommend adding a couple of crossbars in the middle. Then simply mount your gearboxes onto the crossbars and drill a hole out of the side of them (assuming you would use angle for the crossbars) for the shaft to come out of. I would NOT direct drive the wheel on the gearbox. Run chain instead.