We used .125 on our drivetrain this year IIRC, but I've heard of teams using .090. .063 is the thinnest sheet we use on our robot, and I doubt it would ever be on the drivetrain. Make it thicker just to play it safe.
Concerning gearbox placement, you'll definitely want to direct drive one wheel on each side to avoid losing an entire side of your drivetrain to chain failure.
Also, lay off the rivets.
