Actually solid motor mount with rubber trans mount is the way to do it. But if you run all solid mounts, motor and tranny that will cause your problem. Could also be the case already had a weakness or the tranny was tightned without the torque converter all the way in.
You can still use that tranny, just pick you up one of those bell housings that bolt on to the front pump, you just have to cut the front of the tranny case off.