Vented gas cap?

NO (Please) do NOT use vented CAPS!!!!

The first thing is, "what is" the year of the filler neck, "for sure?" There were changes of several caps through the evap/ smog/ carbon can system and you must have the correct one for the filler neck to get it to seal

Second you must have a vent. if the 70 tank you have "had" the evap system on it, there will be extra tube fittings on the tank. if not, there will be NONE. If you DO have the extra tank fittings, the 70 evap system used a vapor separator can in the trunk.

You can "see" all this by downloading the 70 service manual. There is no 75 manual for free unless you have one already that you bought. The 73 manual will be similar and will show the changes that were underway

Frankly I would modify the filler neck to utilize the 69/ earlier vent system. This is documented, again, in the service manuals over at MyMopar