what are some US customs regulations i should be aware of when importing a car from japan?

It depends where in the US. Different states have different regulations. Find a company that will deliver it and do the paperwork for you. I imported a car to Japan from the USA, and had no problems except I had to change the tail light to meet Japan's regulations.

Cheers

The car must meet federal safety standards and your local emission standards. You will need to obtain a letter from Nissan stating that the vehicle meets them. It will need to have your vehicles VIN number. Many vehicles not specifically manufactured for the US market do not meet them. It could be cost prohibitive to have the vehicle modified to meet them if it doesn't.

Japanese cars made for use in japan can't be economically converted to meet the safety/emissions standards that are required for use on U.S. roads. I understand your motives, but unless you have very deep pockets, forget it.

dont buy a jap import they are all clocked