There are reducers for this that do not require any knowledge of the current draw. A simple solution is to use any later model Ford voltage drop unit, salvage yards are full of them and not that expensive new. Something like a 65 Mustang will work, Ford used the same type of 6v gauges up into the 80s I believe, so they all required a voltage dope unit.
HAMB reference:
https://www.jalopyjournal.com/forum/...auges.1134282/