Voltage tester idea
It will work fine. One thing to remember is where the lighter plug is wired. Any load between the battery and the point where the lighter is wired will cause a small around of voltage drop. This is probably at your panel so turning off loads will give you the best measurement.
Also be careful with the exposed wires. You could easily blow a fuse if they touch. If you want to use this often you might wire a series resistor of 100 to 1k. Then it's no big deal if you short the alligator clips.
1979 Contessa 26