There was a part on those '70s carbs called a "dashpot" that slowed the close to prevent high vacuum and rich mixture under trailing throttle. Part of the early emissions controls. Should be able to adapt to your needs. All mechanical and with a bracket. I don't think they are expensive. Just went between the linkage and the stop. Or close to there.
Here is one just to look at. http://www.autopartswarehouse.com/sku/Univ/Holley/Carburetor_Dash_Pot/H19114.html
Might be what you need.


Steve H