My guess on the 60 to 400Hz conversion would be with the use of motor-generator sets or rotary converters (Basically a huge motor shell with the motor and generator windings on the same stator, IIRC).
We had only one rotary converter on our entire base and it was rarely used. I was told that besides the incredibly annoying whine it made (As compared to the "Dash-60" jet turbine generators?!?) it was also incredibly inefficient.
