DJ,
When you say that you found one at a Pick and Pull why not take the gauge cluster and cut off about six inches of that harness. That way you can match up the color coded wires correctly.
There's also a quartz clock option which keeps surprisingly good time. That fills the fourth hole, and looks better than the hole being empty.
I went the factory tach cluster method, but made some changes as opposed to the method in the post 43 link above. It was quite a process and won't go into it unless someone asks.

How about a TACH for a 1989?
A couple of things I did, which I think are worth the time and trouble, are to use gauge faces with the same font and repaint all of the instrument needles with fluorescent orange paint. It makes the whole cluster look like the Blazer left the factory with it, as opposed to having been retro-fitted with it.
While I prefer the newer style font, the tachs were made with the old style and, in my opinion, it looks better if they're all the same.
That actually doesn't look half bad. Paint it black definitely though.