On the setup you have shown (stock) there is no physical connection of the metal in the tranny to the metal in the crossmember. If you take the stock mount out and replace it with a peice of rubber and then bolt thru the crossmember into the underside of the tranny, vibration from the driveline will transfer thru the metal of the bolts into the crossmember of the car, and you will feel it transmitted to the body and steering wheel. Why would you want to do that anyways???
If you put a piece of rubber bolted in between the transmission and crossmember, the rubber would only act as a cushion when the transmission tailshaft moves downward, but you would have no cushion at all (just metal to metal) for upward movement of the transmission tailshaft. A regular transmission mount allows a little bit of movement in all directions, up down side to side forward backward. The engine and transmission are completely rigid, but the frame is a little flexible, so if there's not a little bit of cushion back there, something could possibly break if the frame flexes too much. It might work if you used a long bolt with rubber pads on both sides of the crossmember so that the crossmember "floated" in between a sandwich of rubber. But, how come you don't want to just get a new rear transmission mount?
All the stress and vibration may break the rear housing on the trans. I have seen a fellow do that. The rubber is there for a reason, keep it.
What do you see moving? The transmission or the crossmember. If you see the crossmember moving, thats really bad news. If you see the tranny moving, thats okay---its supposed to. Thats why the rear mount is made the way it is. If the tranny couldn't move a bit, the aluminum housing would break. How much movement---I don't know. All my cars have floors in them.
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