Well, for $20-30 you can buy a new ignition set and matching door locks from most Mustang vendors. That might be your best bet at this point.
Edit:
Oops...I forgot you've got to have the key to put in to change out the ignition lock on the column.....guess you better keep looking for the original key. Some years back you could give the dealership the VIN and they'd make you a new key. However going back 35 years would probably be kinda difficult.