Hmmm? Can't say as I've ever run across this?
I've had many TH400's in many kinds of GM vehicles and have had a few drive shafts built, and balanced. But never run into any kind of Harmonic Balancer? Weights... but not a Balancer?

Sounds interesting, but bulky?

Maybe Quenton, or Tom, or someone might know more on this?

Later - Brett! - 67 Drop Top 455!