The big wire is for cold-start current capacity at low cranking speeds. A HEI unit can, and does draw up to 20 amps or more under certain situations. Check the resistance of a HEI coil sometime, and you'll find the primary winding is between 1/2 and 3/4 ohm. At a cranking voltage of 10 volts, and a slow trigger signal from the pickup coil to allow for full current flow in the primary, 20 amps or more is just basic Ohm's Law.
As speed increases, and the inductive reactance of the primary goes up, the primary current drops noticeably. The wire still has to be capable of "worst case" scenario capacity, however. Ditto for the ignition switch, and the battery wire that supplies the switch. On any vehicle that came out new with HEI, there will be a high-current wire supplying the switch, and very heavy contacts inside the ignition switch, also. The relay is a good idea, as long as it's a continuous-duty type with enough current capacity.
Jerry
