I think this information might be useful. As 
@dirtydeed stated, the flow chart that I made and posted earlier only pertains to commercial use. 
MT has this definition of a commercial vehicle in the CDL Manual found 
here.
Commercial Motor Vehicle (CMV) means a vehicle or combination of motor vehicles 
used in commerce to transport passengers or property if the motor vehicle: has a gross combination weight rating of 11,794 kilograms or more (26,001 pounds or more) inclusive of a towed unit with a gross vehicle weight rating of more than 4,536 kilograms (10,000 pounds); or has a gross vehicle rating of 11,794 or more kilograms (26,001 pounds or more); or is designed to transport 16 or more passengers, including the driver; or is of any size and is used in the transportation of materials found to be hazardous for the purpose of the Hazardous Materials Transport Act and which require the motor vehicle to be placarded under the Hazardous Materials Regulations.
The 
FMCSA website has a checklist to determine whether or not a CDL (CLP) is required. This is the Federal Law regarding personal use. The state of Montana doesn't make any Federal Law more restrictive or prohibitive, so I don't see any issues pulling an 18,000lb trailer with an F-650 for a non-commercial purpose.
How to Determine if a Commercial Motor Vehicle Driver Transporting Personal Property is Required to Use an ELD or Have a CDL
If a safety official stops a driver transporting animals, vehicles or other personal property, FMCSA recommends that the driver explain that the transportation is non-business  related.
FMCSA recommends the driver use the following questions to determine if a CDL, ELD, or paper records of duty status (RODS) are required:
1. Does the vehicle or vehicle combination have a GVWR, GVW, GCWR or GCW (whichever is greater) of 10,001 pounds or more?
          If 
NO, then the HOS and CDL regulations 
DO NOT apply.
2. Does the vehicle or vehicle combination have a GVWR, GVW, GCWR or GCW (whichever is greater) of 10,001 pounds or more, but less than 26,001 pounds?
          If 
YES, the individual 
MAY need an ELD to complete the RODS. However, a CDL is 
NOT required.
3. Does the vehicle or vehicle combination have a GVWR, GVW, GCWR or GCW (whichever is greater) of 26,001 pounds or more?
          If 
YES, the driver 
MAY need an ELD to complete the RODS and a CDL 
MAY be required.
4. 
Is the vehicle being used for the occasional, recreational transportation of personal property for a non-business purpose, such as taking a personally owned animal, vehicle or other personal property to a show when the underlying business is unrelated?
          If 
YES, the HOS regulations 
DO NOT apply. A CDL is 
NOT required unless the licensing state requires it. 
Note: Drivers must verify the licensing requirements in their home States.