I didn't say "don't use spacers." I said the aluminum is soft enough to be easily worked by hand. If the keystock is cut too long, the aluminum can be easily notched. Once you know how do it, it is possible to build a working chasis and drive system in a day with only hand tools. (2 hours is pushing it!)

Mounting the gear boxes and motors on the aluminum takes patience. The wheel mounting plate can be used as a guide. It is notched to fit the square alu0..000000000(that was my son's kitten typing) aluminum. use a center punch to locate the holes then use a drill press. I suggest step drilling (start smaller then go bigger 3/16, 1/4. 5/16) Use washers and lock washers. When the gear box is mounted and snugged up, make sure it still turns freely. Andy binding would be a source for concern. I would mark and mount the motor after the gearbox was in.