50VAC will most likely give you nearer 80V than 70V, depending on how your mains voltage. I'd aim about 10% lower on paper to give a good safety margin. The exact value you should be using for the figures is sqrt(2), which is 1.414... which in theory gives you 70.7V, but in practice the end result is usually higher when there's no load.

I wouldn't bother with a relay for the bleed resistor. Just pick a suitable value that sinks a watt or two constantly. The bleed resistor is only really needed when there's nothing connected, as the drives will do a reasonable job of draining the current once they're connected even if they're not moving anything.