One owner solved the problem of no external fuel filler in the early left gill by carefully hinging the entire gill rather than jumping thru hoops to cut a door in it with all the stock non-existent parts. So his left side gill has no door as was stock for his '71, but hinges up and out of the way of gas station nozzles.
Also note that adding an external fuel system requires a different left side engine cover to go around the fuel pipe. This is not shown in the 'external fuel filling assembly' drawing. Cutting a clearance hole in an early cover exposes most of the big fuel connector while the late one is flared up and around the pipe.
And for unknown reasons, the factory-delivered fuel adapter (p/n 36) that bolts to the body is incorrectly made to align with the tank. The big fuel connector hose is strained in two planes in order to fit, so whatever you build will likely have better than stock alignment and be easier to connect up.
Be sure to use fuel-proof hose, not radiator hose, which will crack within a year from gasoline fumes and deliver gas fumes inside your car.