Jim, doesn't appear to be mileage restrictions. Historic tages do require the vehicle owner to certify the vehicle will not be used for general daily transportation. From the following web site:
http://www.mva.maryland.gov/About-MVA/FAQ/default.htm
Historic:
- To qualify as a historic vehicle, your vehicle must not have been substantially altered, remodeled, or remanufactured from its original construction, and must be 20 model years or older.
- A historic vehicle 60 years or older may obtain a one-time, permanent, non-transferable registration. This vehicle registration does not qualify for specialty plates.
- Vehicles classified as historic certifies the vehicle will be maintained for use in exhibitions, club activities, parades, tours, occasional transportation and similar uses. The vehicle owner further certifies the vehicle will not be used for general daily transportation or primarily for the transportation of passengers or property on highways. You will need to fill out the Application for Historic or Street Rod Registration (form # VR-096).