That line is one of the vents.
The tank was FULL of sh*t so I flushed it like crazy, cleaned with some metal degreaser that contains phosphoric acid, then rinsed, 2 acetone baths, let it dry then coated it with Red Kote sealer (from NAPA).
So it's all back together now (and I replaced the mustache bar bushings while I was in there) and seems to be fine. Feels like it pulls harder and runs better but who knows, time will tell.