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I made a plate that I welded into the neck near the opening. It has the same size hole as the unleaded filler nozzle. The filler is designed to auto shut off when it detects back pressure. There is a small hole in the nozzle. You have to be sure the nozzle fits inside the filler neck far enough to get this small hole inside so it will auto shut off. I made the hole for the nozzle slightly lower than center to allow the nozzle to be pointed downward into the tank. I have a 67 coupe and I have driven it on the Hot Rod Power Tour 6 years now and in 16 different states. It seams to work great everywhere I have been. I can fill at full blast and it shuts off automatically without any splash back.
Oh and it has a 429 so it doesn't pass many gas stations
Great idea! Questions: Did you slide that new plate through the existing opening, then welded it? Or did you cut the inlet pipe, weld the new plate in place, the reattach (weld) the inlet pipe end you cut off? How far down from the gas cap did you go? And how large of a hole is in that plate?