JC Whitney strikes again.  Tachs are less than $20, in the 2in. diameter.
I drilled a hole for it in the bottom center of the Vgon dash cluster,
below the idiot lights.  Available space there is not very deep, but 
black cylindrical body of tach does not look bad protruding 1/2in. or so.
Also tried mounting tach, in its housing, on the steering column.  That
works fine, looks conventional, but gets in the way each time you want
to remove the dash cluster binnacle cover.
B.