I use his A-Body bracket with a mechanical (cable-driven) Jones tach. I had to build an extension to the bracket so the cable would clear the edge of the dash, so my tach sits out an additional 2 inches.
Bearing in mind my tach sticks out further than his stock bracket, it does bounce a little with bumps in the road, but not enough to worry about. It stops flexing within a few seconds, no problem reading the tach. You could put a foam wedge underneath (on top of dash pad) if you need it firmer.
It does not leave a mark on the pad,