If it was a flywheel problem, it would likely be dependant on engine RPM, not MPH. And it would probably continue to increase with RPM, rather than disappearing at very high RPM (if that equates to the MPH you cited). Did the problem appear with the installation of the new flywheel?
What rear axle ratio are you running? The 4th Gens had a problem with the critical frequency of the stock steel driveshaft. Below 70-80 MPH, with the stock rear axle ratios (2.73, 3.23, 3.42) there was seldom a problem. But install 3.73 or numerically higher, and the DS RPM got into critical frequency range at 70-80 MPH, and started to vibrate. It was so bad GM issued a technical service bulletin (TSB) that got some people a new, aluminum DS if the car was still in warranty.
I suspect the number of balance weights will be dependant on the factory detected imbalance of the DS. Does it have a harmonic damper ring at the front of the DS, just behind the u-joint? 4th Gens had one. If all else fails, easy for a driveline shop to check.
Have you checked the tranny mount? .... torque arm front mount bushing?