The two small lines are used for venting or capped if you’re using a vented gas cap.
The larger line in question is for a line up to the filler cap area to allow filling with gas easer. Some would call it a vent line also and it is but it only vents to the upper end of the filler tube to allow the gas to go in the tank.
The coating on the tank should keep it from rusting, painting should not be required. I’m planning on painting mine with Duplicolor undercoating.
As far as mounting the tank check out my build thread (post #252)
http://67-72chevytrucks.com/vboard/s...318544&page=11