is the base of the bevel gear (wide side) rubbing on the gearbox shell? you said it's already too tight for shimming. if it's not touching the gearbox shell, then it's likely that either the bearing/bushing for the bevel gear is not fully seated in its hole. or if it is fully seated, then the shoulder depth of the hole is off spec (poorly made gearbox).
being that you tried a different make of bevel gear and still have the same problem, then it's pointing to a problem with the bushing/bearing sitting too far into the gearbox. a fully seated bearing/bushing, should be either flush or just a hair over the inside wall of the gearbox shell. if the bearing/bushing is sitting more than the thickness of two pieces of paper over the gearbox inside wall surface, then there's your problem.
i had a similar problem with an ares made gearbox for the PTS ACR.
this is how i fixed it.
