I fit a speedhut tach in the stock rubber dash of my 84 Cali II. I needed to take an abrasive disk on a dremel, and slightly enlarge the inner aspect of the rubber housing. You can force it in there without modification, but the larger diameter tach distorts the housing to one side. If you were replacing both instruments, you may be able to get away, with forcing them both in there, with some lube, and it will look "even". Either way, the speedhuts are a great improvement, I also put a speedhut tach in my 98 EV, with excellent results.
Rick D.