What does NHTSA require when importing a JDM car?

Vehicles under 25 years old must be certified as conforming to FMVSS (Federal Motor Vehicle Safety Standards) or imported as Show or Display (rare exemption). For 25-year-exempt vehicles, you file NHTSA Form HS-7 checking Box 8 (over 25 years old), which requires no modifications. If the vehicle is under 25 years and not otherwise exempt, it must be brought into FMVSS compliance or is not legal for road use — there are very few exceptions.