The pick-up has the sender unit attached so if it was installed incorrectly the fuel gauge wouldn't work properly. Also, I think that it can only go one way so that's not likely to be the problem.
So its likely to be the incorrect sender or the pick-up has been modified or damaged. The end of the pick-up could have even broken off through corrosion. The only real way is to remove it and have a look. Fortunately, the tank has a drain plug, so its a relatively simple process of draining the fuel and removing the sender to have a look. Its also a good opportunity to inspect the tank for residues as that stuff invaribly gets sucked into the carby when you let the fuel run low and it will leave you stranded when you least want it to.
Later model Mustangs and Fords had a larger capacity fuel tank and so may have used a different sender unit. Your car might have aquired one of these.
I also know that Mustangs have fuel pick-up problems when going round corners very fast. This might have been your problem as well!
Brad.
