FWIW - I had a stock balancer rebuilt by a guy named Dale in Oregon. From appearances, he does good work. I have put just over 12,000 miles on that rebuild over the last 2-1/2 years. Some of it buzzing along at 2000 rpm in OD running 65 mph - easy duty. Some of it doing stoplight drags and the occasional speed run. The motor has seen 4500 rpm hundreds of times and 5000 rpm more than a dozen times. So far no signs of problems with the damper or the motor.
The crank resonant frequencies that are served up by an old inline six where weight is high and stiffness is low are very low in comparison to modern engines. . . you need to keep that in mind when looking at options.
