Here's the response I got from Marcus:
"You will be fine with 77VDC. The G320 is de-rated to 80VDC and its absolute break apart voltage is around 112VDC. We did this to take into account the line voltage variations as well as component tolerances. Back EMF is not a problem as it gets passed right back into your power supply’s filter capacitor. If you have any more questions, please let me know."
So it doesn't sound like it will be an issue...