Vehicle importation is the responsibility of the U.S. Customs Service. The critical point in vehicle importation is whether the vehicle meets the Federal Motor Vehicle Safety Standards (FMVSS) established by the National Highway Traffic Safety Administration (NHTSA).
Read the Vehicle Importation Regulations, set forth by NHTSA.
Read the EPA guidance for importing vehicles into the US.
Once the vehicle is legally imported, Texas requires that all safety and emissionss components for that model year vehicle be installed and operational. Please check with your vehicle manufacturer (Ford, Honda, Nissan etc) as to what was legal in the United States for that model year.