I use a Mallory Rev-Limiter... same overall diameter as the factory rally-in-dash tach (I could probably make it fit into the dash)... its got an adjustable dial to set the rpm limit... works great and has saved my engine over and over in autocross/road course events. I've got it mounted immediately in front of the factory in-dash tach.
Also, I've occasionally used a small 2-1/16" diameter tach velcroed on top of the dash as it's easier to view being un-obstructed in my straight forward view.