Depends on if you want to spend the money for someone else to do the job or do it yourself. I have a '69 Dart. After all the talk about amp vs. volmeter, I decided to try something different. I had an old amp guage, bought a Sun pro voltmeter from Advance Auto and mated the two. I took the small screws out of the voltmeter. Lined the volt meter face and needle over and around the amp guage, drilled two new small holes for the screws and mounted it on the amp guage. A small amount of rework had to be done where the bolts stick thru the housing, but thats minimal. I went on and mounted it in the cluster. No, it's not original, but I don't have any extra gauges hanging under my dash, and I am able to see exactly the volt reading. The only extra guage I have is the tach.
