OK when you twist those two wires together and use a relay, your going to only need one wire, from that pair, going to the relay. The relay will have two terminals that actuate the relay (control terminals), and two that will allow voltage to flow through when the relay is actuated. Right at the twisted wires, or as close to the switch as possible, put a fuse holder in that line to protect the wiring from the switch to one of the control terminals. Run the other control terminal to ground on the car. Right next to the relay, or too the negetive of the battery. OK the control circuit is done. Make sure you know which terminals are controll terminals.
Then use ADEQUATE wire from the battery positive to an Adequate fuse holder and fuse, as close to the battery as possible. From the other side of that fuse go to the MSD. This wire should be heavier gauge than the relays controll wiring. It needs to be able to supply enough current to support the length of the wiring and the MSD.
This setup will protect everything, keep from burning down the car, put one wire in the engine bay for the ignition positive, and prevent the voltage drop 67 dart is talking about. That voltage drop is important to consider, you dont want your alternator putting out more than necessary.