I would run a new, stock rubber trans mount. Until you get this car lined out.

First, NEVER run a solid trans mount. Chassis flex will usually break it when you start making power. Second, make sure you pull the driveshaft back at least one inch at rest (jacks under the rear end housing, suspension loaded) to engage in the rear end. Much less will jam it into the trans especially if, third, you run more than 4* down pinion angle, and you would need to fix as well. Fourth, your car needs at least a roll bar. I had a full frame Chevelle that ran 12's and was shocked at how much more consistent the car was after installing a roll bar.

i was going to put my two cents in but these guys covered everything so listen to them