Which gas tank?

'71 was when the Feds mandated evaporative emissions canisters or EEC! Instead of venting the tank to the outside the vapors were "stored" in a charcoal canister which sent them to the carb every time you let off the throttle and manifold vacuum increased. Over time the canisters accumulated enough condensed fuel vapor that liquid fuel was sent to the carb and flooded the engine and caused all sorts of problems. When that happened all the lines were plugged and the canister found its way into a dumpster to restore the car to running condition. Either get the '71 tank with 1 vent and plug it or do what the previous owner did and get a '69 tank so you can put things back the way they are with good seams.