If you want to dupe the 70/ earlier non-evap system......go to MyMopar and download a 69 or earlier service manual and look up the vent. They are simple. The problem for the evap vs non evap cars is that the older cars--the filler tube went into the trunk first, then the tank, while the evap cars (generally) the filler tube was inserted from OUTside the trunk
This means if you weld a vent tube fitting into the filler tube, it must be "flush" like say a threaded fitting so you can insert the tube, then screw your adapter of choice into the tube after the filler tube is in place.
I have thought about experimenting with a nut.......braze it flush into the tube, then ream and tap for 1/8" NPT