If all you did to the engine was a carb and governor removal it would valve float before 40 MPH.
The 60 tooth would work nice if driven by an 8K RPM engine.
If you want to keep the engine as is use a 40 tooth sprocket. There should be enough power to get you to 45 plus. And it wouldn't valve float until around 55 MPH, if it can get there.
Not sure why you would use a Juggernaut. The 60 tooth should work pretty well with the stock driver. I use the stock driver with gearing in the 6:1 range and it works well.