The easy and cheap solution is to buy LED bulbs for your Halogen headlamp enclosures. The power draw is greatly reduced and the stock wiring without add on relays works just fine.
During my restoration I was in the middle of switching over to HID headlights for my 73, that required adding ballasts and wiring changes etc. Then LED bulbs hit the market and problem solved. Bought some replacement bulbs for my Halogen H4 housings, ripped out all the HID stuff and threw it away.
I have the original front wiring harness in the car it works great running the LED bulb headlights. And it is as easy as changing a light bulb.