Fuel cap and tank venting

My 73 OEM filler cap is not vented. My tank does not have any nipples, not sure where you are referring too as far as location of this tank nipple. The sending unit has two ports; one supply line and one vent line to the canister under the hood, where its vented to.
The fuel sending unit is a Non-Linear type on all Mopars in order for the gauge to read properly.