Problem I've had with trying to shim certain combinations of gearbox shell /bushes/gears is there isn't enough width in the shell for the gears to run freely. What worked for me is to put the offending gear in with one of the bushes (don't try this with bearings) the wrong way up. Then very carefully tighten down (not fully, just until you don't dare tighten any more for fear of stripping threads) the gearbox screws around it in an even pattern. Not recommended but it should push the gearbox sides out just enough to give the clearance needed.