You may want to check the balance on the flex plate itself in your application since it seems you've eliminated the motor, rear end, and driveshaft.
(I had a driveshaft once that was balanced, but, had a tad too tight a fit into the T400 tail shaft & I ended up with a noisy/failing U-joint. I got lucky on that one and avoided any trans or engine thrust issues!)

Also, in my big block Buick shop manuals (455s) there are service procedures for addressing customer complaints of vibration in the driveline.
After all other causes have been eliminated they suggest placing clip-on weights
directly on the flex plate at various positions until the vibration has been minimized or eliminated.

Perhaps a trans shop could spin up the trans with the flex plate and converter on a trans dyno to do this for you ??

Just a thought