Klunk,
A picture would be useful , as I dont think I can shim the gearbox mounting without transferring the issue elsewhere, as I found out last night. If the gearbox mount is shimmed the Bell housing fouls the drivers footwell.
My other thought was to cut the offending part of the hoop out and then add a flat section that will give the required clearance, this would give a good mounting point for an engine tray as well...
I might look at a couple of shims for the Engine mounts, but I have next to no room as the Thermostat housing on the Raceline water rail is only a few mm from the bonnet when it is fitted now.