Using Old Ammeter

I'm confused about exactly what you are doing, that is, exactly how you are hooking it up

Have you read the MAD article?

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

I'm gonna say "no" that ammeter will not handle 75A

You COULD fool around and add an external shunt, or Bill on here adds a pair of diodes as a sort of shunt.

I chose to say "eff it" and converted my ammeter to a voltmeter. This is not my idea, follow this thread, and read all three pages, as it covers several types of clusters

http://www.forabodiesonly.com/mopar/showthread.php?t=119480