That particular conspiracy law was originally passed to prosecute Confederate secessionists who took over federal property during the Civil War. It doesn’t appear that anyone was indicted under the law during the war and it’s seldom used. Perhaps 50 times since its enactment and usually against radical activists like Earth First.
It’s as illegal to take up arms, seize federal property, and demanding the government relinquish it in 2016 as it was in 1861.