Let's see your A Body Rally Dash in-dash tach replacements...

3-3/8", not 5/8".

It is a pretty standard size though, lots of options in that size from a few different manufacturers. To make it fit you have to remove the tach from the cup it comes in and fit it behind the stock lens. If I remember right some dremel work on the back of the dash panel is also necessary. A few people have done it, @CUUDAK had a good write up but I haven't been able to find it yet.