GM Performance parts sells a 6 3/4 damper

6272221 6.75" Torsional Damper
This cast iron torsional damper (harmonic balancer) is used on most 1969 and later production 305 and 350ci small-block V8s. It is recommended for V6/90º racing engines and V8 applications with limited clearance. 6.75" diameter balancers have also been used successfully on drag racing small-blocks in classes which permit the use of cast iron dampers. The timing mark is 10º before keyway center line.

I agree with Tech, playing with dampers could cause problems.

If you want to dampen (instead of damp) the engine vibration....... just wait till you blow an engine because of harmonics....you'll be P*****