I wouldn't. A good 800W or better is generally considered best for
dual RX-580s... in Crossfire. I'm not familiar with mining GPU usage, tho. They wouldn't be used in Xfire mode, I don't think. If the cards run 24/7 but are
not at or near 100% usage, you may get by OK with that PSU. It all depends on the load the cards will be under.