The 6cyl version of the early Mustangs where an inline6, not a V6.
So the motor turns over..it just wont start. Check for fuel at the carb, could have a clogged fuel filter. With the age of the gas tank in that thing the fuel filter has most likely caught its limit of debris. Also check for spark at the spark plugs. If you have fuel, fire, and compression then it should start.
Is this no-start problem something that has happened recently or is he just trying to get an old classic Mustang running again?