I use 3X plugs with brass conductors on my 6volt FS ignition system. My engine runs super. Why are you concerned about EMI and impedence? The Model A fitted with the FS ignition system is a quantum leap over the conventional point ignition system. I have the FS Zipper distributor which adds to the overall performance of my engine in that it is automatically advancing and retarding under all types of engine loads. This to me makes my Model A fun to drive and eliminates tune ups.
I use non resistor copper cables, no problems at all. The engine starts and runs good.