If his had the ballast resistor like my 58 had, the wire from the ignition switch to the resistor is just like any other wire of it's size. The resistor wires came in a little later I think. So you're right, it should work the relay just fine.
I used the original ignition wire to turn on the main relay when I installed a fuel injected 350 in my pickup. Made life a lot simpler just to have a relay carry the load instead of worrying over the original wiring.
