I don't think too many of us mount our tachs outside of the car. Our dash is perfect for mounting a tach and it looks extremely hickish having it on the outside of the car. It also puts the tach out with all of the elements. Why don't you want it inside of the car? What kind of tach too?
Don't have one yet, just an autometer shift light. Nothing is mounted yet, cause I'm doing a 4 to 8 swap. It's almost complete though. You probably are right though, inside will probably be best.
Inside will be much better bro. You can see it easier, plus where the fox dash cuts down on the right hand side of the driver is perfect for mounting a tach. I always think it looks goofy when I see a car with a flat dash and the tach is mounted on it, it makes the tach look monstrous. So it kind of sucks when you have a 4 eye or SN95. It is still much better in the car though! I think when you have aflat dash it looks good on the steering column, but that's only if you can see your speedo still. What kind are you thinking about getting, and what size?