Tell us what part of the continental US they were exploded over, and how much the power grid was computerized at the time?
There were a lot of tests, and I don't know if any of them were over the continental United States, but this one was near Hawaii, only 50 miles above the surface of the Earth, and had a blast power of 3.8 megatons. (North Korea has barely got a 20 kiloton yield, and H-bombs are far harder to make work.)
It appears to have blown some fuses and circuit breakers in Hawaii, but not done much more damage than that.
As far as the computers go, they aren't likely to be more affected than the old electromechanical systems used in the late 1950s. The EMP is going to be a long wave pulse that will induce high voltages on long transmission lines. It won't have the energy necessary to get at the computers circuits directly.
Over voltage protection on high lines is a lot better now than it was in 1958.