Once your bushings are in, you can begin the shimming process.

First, place the spur gear in the middle bushing.

It will most likely drag on the gearbox wall. If this is the case, start adding shims to raise it upward just enough so that it no longer drags on anything when you close the gearbox.

Once it is high enough, start adding shims to the top to stop the gear from moving up and down when the gearbox is closed. A slight amount of movement is acceptable, but try to keep it to a minimum.

Once you have the gear where you want it, screw the two halves together and test it again. If the gear doesn't spin freely, remove a shim and try again. Try to figure out how much of a difference screwing down the gearbox vs just holding down makes so that you can anticipate how many shims you will need in the future.

Lastly, you do not have to install every gearbox screw for this test. You only need to use the the two closest to the gear or gears that you are testing.