The evap/char system has nothing to do with venting the tank to aid in fuel fill ups. Thats what the 1/2'' vent line is for comming out of the later tanks was for. The early tanks had a prob because this vent hose was absent. The 1/8'' spaghetti lines for the evap system would not flow enough air to vent the tank upon fill up.
Run 3/8'' line if you can then you dont have to worry about any fuel supply problems in the future. Rubber dipped clamps and self tappers to frame hold nice.
I'm only speaking from my own experence....
My cannister line on my '76 got blocked just under the pass floor where it crosses over and up the frame to the firewall...Hardly would fill up at the station....check the filler neck return and it was ok...unstopped the charcoal can line and all was well???
I don't know...like I said"speaking from my own experences"
Also I have over filled before and had gas spill just past the T where they (aux/main/evap) lines meet cause of bad hose9cracked),but would gas go up this far in the cannister line?