Any seatbelt kit wil require some drilling to install them, as there are no provisions for them factory until you get to the '66 models (first year for Federal seatbelt mandate). There will also need to be some reinforcement plates installed, those should come with the kit I would not trust a junkyard seatbelt, as you don't know what it's been through. You wouldn't want it to fail when you need it.
Lap belts would be the easiest/cheapest way to go, and are all that's really required. However, I can tell you from personal experience, they are not the most comfortable if you do have an accident. The 3 point would be much better. You might want to shop around, $300 is a bit steep for one 3 point belt. Juliano's runs less than 1/2 that, and I saw another company, Seatbeltsplus, that were as low as $65. I'm not 100% sure about Seatbeltsplus, but Juliano's has a very good reputation for quality, many vendors use their products.
