Why substitute a useless gauge like a voltmeter for something as important as a temp gauge? Even if the battery and/or charging system fails completely, it's not going to barbecue the engine before you realize what's going on. The original instrument panel has a built-in idiot light for the charging system- - - -what else do you need?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!