In my 71 Challenger convertible, I mounted an Autometer tach in the hole where the factory tach used to live. My car didn't come with a Rallye dash, but I put it in when I redid the car in the early 90's. Been a few minutes, and I don't remember a lot of details, but I took the factory tach out of the gauge cluster, scribed the plastic Lense, and then used a soldering iron to melt the hole out for the new Autometer tach to go through. The new tach fit perfect thru the hole in the woodgrain bezel and the ring on the front of the tach stopped it against the bezel. I put a piece of foam around the tach on the inside of the bezel and used a big hose clamp over the foam and right against the back of the bezel to hold it in place. I ran the wires out of the back of the cluster to hook them up to where each one went.
The tach I used was probably a 3 3/8 round one and may not be as big as what you'd like, but it always worked great for me. I ran the snot out of the convertible at the strip for years, so it wasn't just for looks. It's sitting out in the garage covered up for the winter, but I could get you a pic of what it looks like if you want. It only looks cobbled up to total purists, but a lot of E body guys have asked about it over the years.