Found my dads original 1971 duster!
Personally, I run a trio of Autometers under my dash, oil, temp, and volts(amp gauge bypassed in the cluster) in separate mounting cups, and I love the way it looks :)
What I would recommend, is putting in both a good temperature gauge, as well as an oil pressure gauge, these are the two most critical levels that you want to monitor, and I wouldn't trust either to a 50yr old dash gauge. As @Demonx2 pointed out, these gauges can be tucked unobtrusively under the dash so as not to deter too much from the original look, and like he says, if you are going to go this route, look around, Autometer has lots of different styles of gauges! And dont be afraid to reach out to Autometer to ask questions, there are lots of different types of gauges and you want to make sure you get what you need and what will work best for you. They can be mounted in either a flat panel(easiest way) which comes in 1,2, or 3 gauge style, or like I did in mounting cups in either chrome or black. Ive attached pics of both styles.
Personally, I run a trio of Autometers under my dash, oil, temp, and volts(amp gauge bypassed in the cluster) in separate mounting cups, and I love the way it looks :)