You need to isolate where the fumes are coming from. They could be exhaust fumes, gas tank fumes, fuel line fumes or fumes from the carburetor:
Gas tank fumes: old Mustangs have a fuel tank with a thin top in the trunk. If someone dropped a jack on the fuel tank, it can put a pin hole leak in the tank and consequently spread fumes into the passenger compartment. Clean the top of your fuel tank and then put duct tape over every square inch of surface area. If the gas smell goes away then replace your gas tank.
Carburetor fumes: if the float levels in the carburetor are set too high, fuel will sometimes slosh out of vents and onto the carb top areas. This leaves a nasty fuel smell that seeps out of the underhood area and gets sucked into the cowl vents. Use masking tape to seal the seam between the back of the hood and the cowl grille (cover the cowl grille). If the smell goes away with the tape on, then the fumes are coming from under the hood. Inspect the top of the carb and manifold for evidence of fuel leaks.
Gas lines: check every inch of the gas lines to make sure there's no pin hole leaks. Old lines get corroded and sometimes leak fuel. Duct tape any areas found and re-test for fumes. If the fumes go away after taping the lines, replace the lines.
Exhaust gases: A richly tuned carburetor can result in raw gas being expelled into the exhaust system. If there are leaks below the body and before the rear of the car, these can be smelled as if they were in the passenger compartment. Inspect exhaust system for leaks. Check exhaust pipes for evidence of rich adjustment of the carburetor (black sooty exhaust). If you find leaks, repair them. If you have black sooty exhaust, retune the carburetor to a leaner setting (should be grey looking exhaust).
Good luck!
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