You definitely have a venting problem. If fuel is kicking back out of the filler neck when you fill the tank then the vent is blocked. Crawl under the car and disconnect the vent connection at the tank and blow air from a compressor into the disconnected line (also disconnect the charcoal canister). Have a helper listen at the other end of the line in the engine bay. There is an overfill limiting valve in the vent line, it may be plugged with gunk. You also need to blow air (with open gas cap) or run a piece of bailing wire into the vent inlet nipple on the tank and make sure it is not plugged. If you can get air moving freely through the vent line reconnect at the tank but leave it disconnected at the charcoal canister. Try filling your tank, if this works then I would place a simple filter over the end of the vent line and call it good. This is all assuming you are using the correct 1976 fuel tank and filler tube. May want to ensure fuel is not leaking at the filler tube to tank gasket. If it is you need to replace it.