There are many factors which could cause this condition. For 1973 with the ECS (evaporation control system), you should have a single line from the tank which goes all the way up to the engine compartment and connects to a charcoal canister mounted at the right front corner. First just make sure your system is connected correctly. You say the pipe and rubber hose were "just dangling." Is your charcoal canister there? There should be the line coming to the canister from the fuel tank and a vacuum hose from the top of the canister to the purge port on the carb. (Can't remember if the '73 has a third hose off the canister.)
As an auto technician for 30 years, any time I would diagnose a fuel back-up situation on a modern car, I would look for these things: A blockage in the vent line(s). Any kink or blockage in the vent pipe or rubber connecting hoses can cause your condition. Believe it or not, I've seen spiders build nests inside the vent tubes and cause a blockage strong enough to make fuel back up when filling up. Compressed air through the tube isn't enough to remove it. It has to be flushed with water with the tube off the car. A faulty purge valve. On the '73, the purge valve is actually a port above the throttle plate on your carburetor. If your carb is not an original, chances are the throttle plate (which is your purge valve) is incorrect and you will have to get a separate canister purge valve. Failed charcoal canister. I've seen charcoal canisters break down due to age and fail, causing the same problem. Failed or incorrect fuel tank. This would be the last thing to consider but can be an issue.
If anyone else out there has any suggestions or sees something I have overlooked, please chime in...
