You can buy a gizmo that does what you want:

http://www.hawksthirdgenparts.com/inc/sdetail/17491

It aint cheap and it aint pretty. Make sure your pulse rate is
compatible with the box....there are three GM pulse rates and
who knows how many for other brands

The 231 was available with a mechanical speedo in the earlier models unless someone put a SYE kit in it. Are you jeeping this thing?

mike in tucson