Good luck with that. When I got my 51 the gas cap didn't vent, you could hear the tank oil canning as the sun heated it up. Bought a new one, did the same thing. Turns out the new caps have some sort of pressure valve to vent both pressure and vacuum. The original was essentially just a hole to atmosphere. Had to weaken the spring retaining the pressure to get it to stop oil canning the tank. Maybe an NOS one might work though.

As for the vent nipple, I think that's an aftermarket thing since the tank in my 65 has the same vent setup as that 70 one the OP posted. No nipple above the sending unit.