I'm sorry, but I don't think that this is quite correct (may be how it was worded). The tank and vent system that is used in 1971 on (I believe) all Darts is the same as the 1970 California emissions tank and has the weird internal tank. All non- California emissions tanks in 1970 just had one vent that runs from the filler tube near the quarter panel through the trunk floor gasket into the frame - not connected to the tank at all.
If you look at the tanks in post #33 the right and left tanks are the same tank, except the filler tube hole in the tank on the right is not in the center of the rounded flat place as it is in the tank on the left. I had this issue with one of my 70 darts and ended up going to carlisle - looking at the tanks that they had with them at the spectra display, bought a specific tank with the hole in the right place and had it shipped home to Washington state. It is possible that this is a quality control issue to some extent .