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.