Here's what I did.
http://www.73-87.com/garage/gauges.htm
I used a mechanical speedo because an electric didn't come in the Pro Comps gauges at the time. They do have one now. I'm not sure, but I don't think there is much you need to do to convert to electric. But, the electric speedo is a lot higher than a mechanical.
I've go $450.00 in just the gauges and wiring harness. I used a mechanical oil pressure gauge too. I also decided to use a braided stainless steel oil pressure hose, just to satisfy my peace of mind. That was another $50.00.
Oops, I forgot to say, the $450.00 doesn't include the two gauges where the radio used to be. They are a vacuum gauge and a tranny temp gauge.