A company called Motorex, based in California, used to import R32, R33, and R34 Nissan Skylines into the US. They actually went through the full process of importing and federalizing the car; they kept them right hand drive and added US emissions equipment to make them 100% road-legal.
They've since stopped doing it, and I heard that they fell foul of the law on some stupid technicalities. Apparently, when you go through the process of federalizing a car, you get approved to convert car model X, and only car model X. Even a different trim level of the same car requires its own federalizing process, which costs a fortune. Motorex sold a variety of trim levels when they apparently had a permit to convert only a certain trim level or something like that. A shame.
There are a lot of rules, and anything less than 25 years old must comply with the full set of USDOT standards in place when the car was built. Emissions regulations are retroactively applied to some vehicles stretching back into the 60's.
There are some loopholes. Race cars are exempt from most of the BS, since they are not road-legal anyway. Cars older than 25 years are easier to import. There is a "show and display" category whereby you can import cars deemed technologically or historically significant (but non-compliant with USDOT standards) and drive them on the road so long as they clock less than 2500 miles per year.
I think it's all stupid, in the sense that Europe, Japan and the US all have sensible safety/environmental standards, but none of the countries recognizes those of the others.
The list of cars I'd want to import is endless...Lancias, Alfas, Euro-spec BMWs and Mercs, TVRs, the Lotus Carlton, Alpines....ugh. Better not to think about it.