66 Barracuda Formula S - Performance indicator instead of Tach?

For what it's worth, I had a 66 with the vacuum gauge and no tach, BUT, it was a 2 barrel, 273 4 spd car. I swapped a commando engine into the car and also replaced the rear springs with HDs. I didn't put Formula S badges on the car, but if it were still around, it could cause some head scratching.