Put a .3mm shim in addition to the provided spacers in the rear diff cup. Diff cup being the key word. That solves the gear mesh in the rear diff. The front diff is shimmed appropriately.

Lower the servo by 2.5mm and move it as close to the center of the chassis as possible. Use a normal servo horn and the steering travel will be correct.

The tank is mounted with too much vibration for proper pit stops (erupts fuel when pitting), so dremel the tank slightly so it's clear of the carry handle (roll bar). This tank is mounted on 3 points, make sure the tank is fitted loosely to the radio tray (enlarge the tanks holes slightly).