For the 1963 and up, the ammeters are very low current meters. Unlike the 1962 and down which are true meters.
On the 63 & up ammeters, 3 amps will smoke the meter and even melt the plastic frame inside. In truth, the ammeter is the fuse. It's a very good idea to install a 3 amp quick blow fuse in one of the lines. (Does not matter which one). This will protect your meter from blowing out. ** I personally would first install a 1 (one) amp quick blow fuse. If problem persists, then try a 2 (two) amp quick blow fuse.