Figure the amount of power the lights and relays will use, plus the lengths of wire and the operating temperatures (moderate temp in this case behind the radiator support). I would recommend THHN or XHHW
insulated wire good to 170F temps.
Headlights on low pull between 35w to 45w (3.2A) each bulb, high beams about 65w (4.8A) each. Two low beam bulbs would draw 6.5A. Two high beam bulbs pull right about 10A.
My recommendation would be to pull power from the horn relay and not the battery. When the engine is running, the alternator provides power directly to the horn relay over 10g wire, then to the battery through a second 10g wire (on the 1970 model car).
Battery to high beam relay: draw of 10A for two high beam or 20A for four high beams. I would recommend wire for the battery to high beam relay of 10g or 12g up to 10 feet in length.
Battery to low beam relay: 6.5A for two low beams. Recommend 12g or 14g wire.
Relays to each headlight lead, low or high beam: 14g wire. This size wire will support the 5A each bulb can draw. The factory wire to each low beam and high beam connection in 1970 was 16g wire, smaller than my proposal of 14g.
Between the power source and relay, for circuit overload protection I suggest a self-resetting circuit breaker instead of a fuse. Low beam relay can be protected with a 20A circuit breaker such as Littelfuse 20 Amp Circuit Breaker UCB20BP ($5), while the high beam could use a 30A breaker Littelfuse 30 Amp Circuit Breaker UCB30BP ($5).
Headlight sockets could be replaced with Dorman Conduct-Tite Headlight Socket 85810 ($3) or equivalent.
I did this headlight relay conversion in 2007 and have been happy with its performance.