Branda '66 Shelby Tach Fitment

Are you mounting in the Shelby style pod? If so I did this today but am using a Sunpro tach. If you are mounting them in the pod I have had great luck getting the gauges mounted and then using expandable foam to firmly hold them in place from behind, I was tired of my old ones rattling around. I then put tee nuts on the inside of the dash to hold the the whole thing securely in place.

Just make sure you mount it in the right place, or the dash will need to be really crushed flat to keep it from shaking while you drive. check out some pics of original Shelbys for the correct location. If being original isn't a concern, I have seen guys make a small bracket that extends from the tach base to the speaker grille, that way they didn't need to drill the pad.
When I bought my original Cobra tach at the Ford parts counter, it came with the dash bracket and a column bracket. After trying it on the dash, (without drilling holes) I opted for the column mount, it just seemed easier to watch the tach there.