I would recommend getting a new lock set from one of the classic Mustang venders.
At a minimum, I would get the ignition lock. The door and trunk locks are easily removed and you could take these to a locksmith and have new keys cut or the door locks could be recoded to your new ignition keys.
The entire ignition switch comes out of the dash as does the door locks and trunk and glove box locks. Either available as a whole set from Classic Ford parts sellers or just have a Locksmith over to make you up a couple sets of keys and an extra one to hide.
I would suggest taking the locks out and taking them to a locksmith. This is a good deal less expensive than paying for mobile service, and is likely less expensive than replacing the locks. Plus the fact that aftermarket locks may very well be lower quality and may corrode quickly.
Any locksmith can make a matching key in only a few minutes. My friend lost the only keys to his 66 Ford and the locksmith made a set of keys from the passenger door lock (on the car) in less than 2 minutes.