Used a second filler neck for any older vehicle. Mine was from a '60 Ford truck that I had laying around. Decided where I wanted the new neck to join at and cut the end to fit the proper angle. Got that shaped to fit the curved shape before cutting into the original. Used the new neck then to draw a pattern on the original neck. Removed the neck from the car and used a dremel to cut the oval hole. I brazed the two together and water checked it for leaks. Then, in order to get it in the car, I had to cut the top off of the original neck. I cut the existing rubber hose in half and got a couple more clamps to get it all installed. The cap I used for the inside cap was originally a vented style since I could not find one to fit that was non vented. A bit of JB Weld was used to plug the vent hole on the inside of the cap. You do not want fumes inside the trunk.
If doing it again, I would make the new neck shorter so the cap sat closed to the original neck tube. I also would run a small hose (with two small steel nipples)from just under the new cap to the old upper part of the neck. At times, fumes would collect in the new cap area and this would eliminate that. It still actually vents out of the original cap so if overfilled, you can still get a fuel leak. Sealing the original cap and installing a vent tube out of the trunk floor would stop that.