There is a "U" shaped bracket, on the back side of the tach, that holds the tach tight to the dash panel. There are two studs that go through two holes in the bracket. Remove the two nuts (5/16 or 3/8,as I recall) and remove the bracket. From the backside, push the tach towards the driver's seat. There is a stud on the backside of the tach housing, and as I recall, it's the ground wire for the tach light.You'll need to remove it.
Sounds like you do not have a shop manual, nor a parts book. I'd strongly encourage you to obtain both. It will make your work on an Avanti much easier.
JB