www.redlinegaugeworks.com will do this same conversion. He can also convert your amps to volts. And convert your temp gauge to show actual numbers in original type script with a calibrated temp sender (new replacements have the wrong resistance).
I will also be running a set of aftermarket oil pressure, water and trans temp gauges just under the dash next to the underdash A/C vents.. The factory amp gauge is being converter to volts. Redline did my 70 gauges. My trip meter still doesn't work right. I don't think its a problem specific to Redline though. I believe its some kind of a factory issue....maybe. My buddy posted about his '70 doing the same thing. Everything else they did was nice! Here's a shot of the '70's interior. Tee blue tint is the windshield factory tint with aftermarket tint over it.