If you do a 60 Hz mod, then you *must* either replace the oscillator or use an RGB cable. If you add a 50/60 Hz switch and use composite video without replacing the oscillator, you'll end up with a wacky rainbow pattern.
For 60 Hz, you need a 53.691 MHz oscillator.
(If you use an RGB cable, then this doesn't matter; just the switch will be enough.)