I believe alot has to do with the overall condition of the car and perceived value. Perkins cut up a 800 mile 69 coupe and on the other spectrum, I have seen 65K+ mile mustangs that looked well preserved.
I think some factors to consider include: is the Mustang something special (a 69 6-cyl coupe in an ugly-a$$ color with a crappy green interior is not special). Has the mustang been repainted, partially or completely. What is the condition of the body (and top), interior, engine bay, and underside (I know one of our MCA assistant head judges is an expert at removing grime and undercoating from one of his unrestored 65s).
Basically, mustangs with replaced body parts, significant repaints, worn interiors, any rust will move the needle away from conserve to restore.
Regards,
Pete Morgan