I'm adding a tach, too, and after pricing all the available options I decided to go OEM. The sensors are $75 from the dealer$hip and then you have to find an instrument cluster with a tach...common as hen's teeth for 83-86 diesels.
But I just bought
this today, and I'll be hacking it into my existing cluster. Either that, or I'll make a housing and dash-mount it. Total cost: $108
Or, you can get a MiniTach...costs a bit more and I don't like the LCD display idea. That was my second option, chose OEM. Not sure what others here have done.