That sounds like my dream trip. One day when I'm retired. I did have few opportunities about 5-7 years ago while my son was stationed there. I was never able to get the timing down. My loss, but probably saved me a TON of money.
I know Tariffs were brought up several times. HTS Code 9706.00.00.00 is confirmed as the correct U.S. tariff classification for importing antiques over 100 years old, including Japanese swords over 100 years old. You should be fine as long as you document correctly for the swords trip home.