I would buy a new set of locks.
These new locks can be recoded to suit the key number in your owners manual within keeping everything matching... or install the set as is.
I'm trying to steer you away from rebuilding your original locks as the barrel casting will be worn, while the barrels can be repined they won't have that fresh feel.
Even with a key cut to code (not a duplicate from a worn original) the operation could be hit or miss.
On a early Mustang I would also purchase a ignition housing as well.
WD40 can be used to wash your locks out then followed up some type of dry spray to obsorb the WD and give a light coating (In Oz we have Shelley's Dry lube or Zip ease)
While you can use Graphite (I wouldn't- Sorry caspian65 and jwc66k
) as it will end up all over your hands in your pockets every time your unlock your car.