ntallis

Alternative gearbox mounting that can be used is a Land Rover engine mounting, this has the advantage of being stiffer than the o/e mounts. A stiffer mount can help particularly with an overdrive gearbox due to the additional weight of the overdrive. It is important to adequately support the rear of the gearbox, because if the rear of the 'box is allowed to drop, the alignment of the propshaft front/rear joints will not be equal and this will also generate a vibration through the body.
If the vibration you are experiencing is a harsh metalic one then replacing the two circular supports for the centre prop bearing will definitely help, also a stiffer rear g'box mount will help this as a soft mount can result in metal to metal contact between the g'box and the x member.