You can take the gauge cluster apart, and just put in the tachometer you want. This will allow you to keep your speedo. Unless you want to pull the speedo apart, and you can just change that section of the gauge cluster.
It's possible to "adjust" the odometer to read whatever miles you want(or km).
When i installed the turbo engine my tach did not work. I got a tach out of a turbo car. I pulled the dash and replaced it and it work without any other adjustment. And i have a volt meter still and a aftermarket boost gauge.