jb,
When I first built my bracket Camaro many years ago, I broke two TH400 bellhousings in less than two months

My trans guy told me to get a driveshaft made. That took care of the problem. Put over 2000 passes on the car after that and never had a single trans problem. I never felt any type of vibration at 7200 rpm's but the old driveshaft must have been out of balance or out of phase or something.
By the way, I had the same setup as you, solid engine mounts with a rubber trans mount. The car was a full chassis with cage so there was not a lot of flex.