It's not the state level that matters, but the federal level. If you just attempt to import a 450 diesel like I import JDM cars, U.S. Customs and Border Protection will likely be mad at you. All of what I mentioned above are federal requirements, not state. The states rarely care so long as you have a title.
Now, technically, you can just drive one across the border, and tell border patrol that you're just driving south. I personally know of many cars "imported" that way. It's not legal and, if the feds find out they reserve the right to demand its exportation or to crush the car at your expense, even after the car turns 25. That's because a car isn't automatically legal at 25, you still have to do the importation.
It does happen, sometimes. I know a guy who got a Holden Ute across the border. The feds later found out, and demanded it be exported or crushed. It was exported to Canada. Another guy imported a Suzuki that was of legal age, and it was deemed illegal because it had a paint job similar to the old VW Bus, and VW demanded it be crushed. My friend legit spent $9,000 on nothing.