I did some research after Jerry mentioned the shunt. It turns out you can use an original amp meter but it takes more effort. You have to play with finding the right shunt for your application so that it flows most of the dangerous amperage around the gauge while allowing the right amount for your gauge to show. This is done by changing the length or gauge of the wire you use as a shunt.
The other issues seems to be the increased hazard of fire by running the wire into the cab, where as the volt meters just have a small wire that enters the cab and is not the entire amount carried from the alternator. This is simpler and apparently less risky, however I am very inclined to want to keep as much original in the car as I can and as little extra out.
