1970 Chevy El Camino Restomod
This does not appear to be the problem. The vibrations occurred with the old torque converter. When the shop decided to pull the motor again to check the flexplate for cracks, I asked them to just replace the flexplate and at this time they also installed a new torque converter. The vibrations occurred before and after this change, so this does not seem to be the problem.Vibration: Before the engine removal, when you did your last 1/4 mile tests, is it possible that one of the torque converter turbine blades bent or one broke on one end? This might change the pressure and maybe the amount of fluid (weight) in parts of the spinning convertor. This might cause vibrations. I don't know how commonly this occurs.
I think you have a TH350 or TH400, so I don't think you have a torque converter clutch, so this would not be TCC shuddering.
Might not be the best reference;
3) Vibrations and Shakiness
If you accelerate your vehicle past 40 mph and experience a lot of shakiness and vibrations in the cabin, then you probably have a bad torque converter. It will feel as if you’re driving on a gravel or bumpy road. You cannot let this problem carry on for too much longer because it could lead to even bigger problems with your transmission or engine.
5 Symptoms of a Bad Torque Converter in Your Car - Autocartimes.com
Thanks for the idea, though.