There are considerably more than 400 silos. Most of the silos are dummy silos, which contain a working Minuteman III but it has no nuclear payload. The missiles that have the nuclear payload change. To guarantee that you take out every one of the 400 Minuteman III missiles that have a nuclear payload you would have to hit every silo. Which is quite a few of them. Our SSBNs while difficult to track are not impossible to track, and we have only 14 of them.
If we hadn’t backed down, there would be MX (Peacekeeper) missiles, with 10 warheads each, wandering undetected around the U.S. in rail cars, and buried at undisclosed locations.
The MX was called that (”Missle X”) because it was originally designed to be a moving target, or a hidden target that the enemy couldn’t hit because it couldn’t be found. Stored in rail cars, and buried lengthwise in shallow trenches, it was in a big tube which popped up on command, and spit the missile out with a steam charge before firing the first stage.
It was a beautiful system. The USSR was scared out of their minds, but we capitulated (Thanks Jimmy Carter...), limited it to 3 warheads and promised to put it in stationary silos. Had we moved with the original design, we wouldn’t even need the silos, and this discussion would never have been started.