A 68-72 tank will not fit. However, a 73-74 tank will fit 4th gens.
The tank itself stayed pretty much the same from 73 to 79. The filler necks were a little different with 73-74 not having a restrictor for unleaded gas nozzles and a double-tang twist-on cap instead of a screw-on cap. There was also a slight change in the routing of the vent/vapor line that took place sometime around 75 or maybe 76. The early ones have the vent line coming out the top of the tank and leading over to the front driver side corner where it attaches to the vent line that runs up to the front of the car. The later ones have the vent coming out the top of the tank and leading over to the passenger side of the tank where it attaches to the vent line that then crosses over to the driver side and runs up to the front.