What leitmotif is getting at, is that some ammeters have an internal shunt and some use an external shunt. If you find a shunt to buy, it will be for an ammeter with no internal shunt.
I do not have a wiring diagram for a '57, but I suspect it has an internal shunt.
External shunts have much lower resistance than the .3 Ohm that guy used. He is actually using a second shunt, in parallel with the meter shunt.
Remove all wire from your ammeter and measure its resistance. If it does not have a shunt, the ohm meter voltage will peg the ammeter. Oops.!!! If it has a shunt, it will probably read about .2-.4 ohms and no meter movement. Short the ohm meter leads together and subtract that static reading to get the correct value.
If you think you are going to increase the current through the meter by that much, then you need to be more concerned about the size wire and the quality of the connections under the dash.
They stopped using ammeters on cars as more and more accessories were added and current requirements increased. Chrysler stuck with full current ammeters up into the '70s and started having problems with overheating the connections in the firewall connectors.
One way to decrease current through the ammeter is to use an external shunt type ammeter as GM did on my '67
http://www.geocities.com/chevroletc1070/v8engcomp.htmlThe 12R wire that goes from the junction block at the battery, over to the wire splice (junction) below the alternator and the regulator is the shunt. The 12R wire is the shunt. There are 1 Amp inline fuses connected at both ends of the 12R wire. Look for two small grey, inline rectangular boxes in the diagram. These fuses connect to the ammeter. The 12R wire carries almost all of the current and the meter only milliamps. If the 12R wire were to be damaged, one of the fuses must pop or the meter is toast.
