if it jams it means there are too many shims and the gearbox is squeezing the gears.
reduce shim thickness, put the box together and try to move the gears from the bevel gear with a big flat screwdriver: it should be relatively easy and smooth.
were the spur gear seems to touch the box, there should be a very thin+wide shim.
Also you need to adjust shims so that they optimize contact in between gears, not only gear vs box: if the spur gear gets to be higher, then you need to lift the other gears accordingly.
a good shimming will require you to take apart the box several times especially if it's new for you so don't worry and go back to it
