The fuel gauge wire will be a ground. You can test with a light any wire close by and if it has power, put a ring of tape over it to prevent a short. The one/ ones that don't have power, put them to a good ground and watch the gauge, you'll know when you hit it provided the gauge does work.

I don't know how the fuel lines ran but I was thinking that maybe one of those shops that makes the line kits might have a diagram as an example? You could also get the sizing too! But 3/8" is good, they aren't racing it I'm sure so it'll be overkill.