Just wanted to share my experience with this tach in case anyone was thinking of buying one. Mine came from OPGI.
It's CRAP, really cheaply made. Basically unusable as it is. Could not even mount it as the holes were off, and no way to make new holes. In my efforts trying to make it work I kind of made it unreturnable. So then I ended up gutting it and butchering a 68 clock to use the case, then I mounted the guts of the tach in the clock housing.

http://www.ebay.com/itm/151421264095

 

I feel your pain Brother, what a piece of junk. It looked like they glued popsicle sticks to the sides for mounts. I was very disappointed.

 

On OPGI:

Imagine my surprise on getting brand new brake wheel cylinders for one of my rides (a '55 Cadillac) from OPGI only to find they were made in China!

What is "original" about that, one wonders?

 

The tach from GM Sports has been discussed many times on this site and everyone who bought one felt the same way. They also make them for 71/72 and the plastic housing was also a point of concern. I'd still contact OPGI and tell them your experience over the phone. They might be willing to take it back. If nothing else, call GM Sports and complain about the quality.

 

I bought a sun tach but im not doing a exzact restore had a clock in the hole didn't work wrapped some tape around tach till it fit snug and used the wireing and lighting from the old clock