To what Royse said I would add this:
It took a long time for me to come around to a 3 wire system. 3 wires = more complicated than one wire right?
I have later tractors that used an idiot light for the oil pressure and charging.
That big red eye will get your attention a lot quicker than a gage will.
There is a website that has about the simplest diagram for a 3 wire system that I have found.
It is for converting a Land Rover but the diagram is very simple to follow and it has some interesting stuff about 10SI and 12SI alternators. Click the link below.
Two other things: I prefer a volt meter over an amp meter. I think volts tells you more. And as Bruce suggests skip the resistor block and use the 'real' 12 volt coil.