Interesting, okay I just assumed it went into a fuel pump for a return line. Your diagram suggest it's a lower vent line (in my truck there are two lines coming out of that floorboard grommet) but as you can see in the second set of pictures it used to have a clamp on it at some point. There is only one line into the pump then hardlined almost all the way to the carb. It seems silly they would go through the trouble of venting all the way to the engine bay with hard-line just to leave it open, unless this is what the charcoal canister would be for. I've never heard of it but will do some googling. I think I'll just replace that section of line and be careful not to fill the tank too full for now.
I appreciate your help, thanks