Not sure what you have going on, but let's start here:
Remove all the stand off stanchions from the Tamiya gearboxes.
You do not use them at all. The Tamiya Jagdpanther gearboxes
sit flush to the gearbox mount.
Remove all the Tamiya gearbox stanchions, you do NOT use them! The gearboxes sit flush to the gearbox mount brace,
My Tamiya Jagdpanther Build.