Don, the meter movement is protected up to the amperage rating of the meter. No more than that. The problem we have when making suggestions here is that if you are wrong and someone followed your advice, they are homeless and carless because their truck burned down in the garage. We want to err on the side of caution when giving advice to others.
Though its true that a 6 volt power source (battery) drives twice the current as a 12 volt, many factors are in play when it comes to power output. Circuit resistance, design of the devices, and best practices come into play. The problem with suggesting that 6 volt wiring is stouter and can handle twice the current is that its 50 year old stouter wiring! Maybe hanging on the terminal by two strands! Since most of the wiring on most of the trucks I have seen is completely shot, I wouldn't rely for a minute on wiring that old unless it has been thoroughly checked. I am sure you do that, but others do not. That IS their problem, but in this case it can also be the problem of their wife and children.
In the very least if you have more than a 50 amp alternator, do what HRL suggests and cut a piece of radiator hose clamp to go in between the poles on the ammeter. It was a great suggestion because it at least gives the extra amperage an alternate path (which is what a shunt is).
