In 1970 Mopars didn't have charcoal canisters to filter the fumes from the vented air. Even the California emissions didn't have them, only a vapor separator to prevent fuel from spilling out the vent and the fumes were released into the crankcase.

Warming weather can cause more of an odor.

My suggestion is to check everything about the fuel tank for any wet spots and then fill the tank.

Give it a little bit of a run making sure the slosh the fuel a bit and look for wet spot/leaks again.