Gauge AMPs verses Volts

I think the reasoning is that many old amp gauges did not use a shunt and instead ran full charging/discharging voltage/amperage through the gauge. It's a safety issue.

seth ic correct with our dodge's from the 60's but don't know about the ford or gm,,,

I don't think an AMP gauge is "bad" by design. It shows what you wrote - Batterie charging or discharging by approx. what amperage. That's good - but.
It doesn't really help, if connections, splices and such are bad.

Volt shows the current voltage at that special point where it is measured. Voltage shows you charging/discharging as well (being below "normal battery" (approx. 12.6V, depending on battery status) is discharging, above being charging).

I personally like the fluctuating amp gauge in my classics. Nonetheless, I carry a (more or less hidden) voltage gauge on all of them. Charging with 40 amps for over an hour -twice on a single day- worked flawlessly with a 50 year old ammeter (70Ah battery was completely depleted) - because I meticulously take care of proper cables (gauge!) and clean connections. That's important. Very important.

Both in conjunction and knowledge about interpreting their readings is helpful on the road, to early address problems in the electric system, as they can mount to total losses easily.

Dodge A-Trucks/Vans are just a little easier on this topic, as the do not have the problematic bulk head connector of the usual trucks and passenger cars. That's a single point of failure, if not taken care of proper connection and no corrosion. I don't know about G/E-Vans, though.

here is something i posted in 2013

https://vintage-vans.forumotion.com/t37936-removing-that-amp-gauge