It all depends on the quality of the car. How are the floors including trunk, frame rails, torque boxes,whell houses, and rocker panels? Are they solid?
If they are covered in undercoating beware, thay are hiding something.
You need an experienced person to put the car on a lift and inspect it. If the car is good the seller should have no objection to doing this. Depending on the condition I would guess between 3,000-8,000 dollars.
1967 Mercury Cyclone GT
1970 Mustang Mach1