There IS NO return line. You have to add one, re rework the vent system Read my earlier post. Either you have an old style (69/ earlier) vent up in the filler tube, or you have an evap control system with the line going up to the front. READ THE shop manual
If you don't have the evap system, you will have to add a line to the front
If you DO have the evap system, you will either have to convert the vent to the old 69 style, or else add a second line, because the line up to the front IS the tank vent on the cars with evap controls.
As I said earlier, SEVENTY is the split year. Normally, CA state cars got evap controls, 49 states got the "69 style" vent
The other things you can try is to examine the fuel tube routing up by the engine / exhaust. Maybe someone changed the routing. Maybe you have headers "made it worse". Try building a heat shield / glass insulation.....you can buy Summit, etc
Consider installing a rear mount pump.
IT ALSO might be "other problems" YOU NEED to check it out
Is the tank vent plugged? Try removing the cap
Is the tank pickup sock plugged? Supply hose from tank to tube collapsing, restriction in the supply tube?
BAD fuel pump? Again, the service manual has procedures to check out the pump