I just installed this relay into my 1969 Chevelle:
I say "installed" as it didn't require any splicing -- plug and play. Okay, that's not entirely true as I did cut and re-splice the far headlight socket, but that was because I was picky about the harness routing and wanted to go through the stock holes in the front end. It took me a couple of hours as I went slowly and did some trouble shooting for other issues. To do a second one would take me about 30 minutes. Just make sure you have a good ground or else your lights will do all sorts of spooky stuff.
The relays are amazing. I have 10-year old Sylvania Halogen bulbs in there and the light output is close to double when driving at night. The interior and dash lights also got a boost. I know the butt-o-meter isn't reliable, but the car seems to run smoother -- that may just be false perception. I put the relay harness into the low beam circuit with full intention of putting another separate harness into the high beams, but decided against it as so much power was freed up by the first harness that I didn't feel it warranted the extra cost and clutter. The brights -- in the stock circuit -- are also significantly brighter. I set up everything up during the day and was a little disappointed in the effect, but when I turned the headlights on at night and drove the improvement was obvious and immense.
You will still have the stock lighting of course and won't have the modern, sharp-edged beams that a lot of people go for. The light will be diffuse but much, much brighter. For about $40, it was a huge bang-for-the-buck upgrade.