The max power draw of his build is 663 watts. There's no way it can pull more than that unless you highly overclock, and even then, a 750 watt PSU will be fine. 100 watts is nowhere near cutting it too close
Exactly, to hit 663W he has to tax EVERY component to 100%, with literally will never happen unless he's running some very specific benchmark tests like OCCT's PSU test. Which, spoiler alert, you shouldn't run unless you are extremely confident in the reliability of your PSU and/or you are testing the max power consumption of your system.
Overclocked that system can eat over 700w. If they ever add any other parts later, that goes up. Using 700w of a 750w card is not safe, it's stupid. Spend an extra $10 for a bigger psu. Running too close to rated power is taxing to a psu and causes failure over time.
Show me a test where a 2-way 770 OC'd build needs a 1000W PSU. Everything I've seen points to 750W being way more than enough, after taking into account load degradation headroom.
And by that I mean a 30% gap to full load. I don't know why 750W is so unthinkable to you, unless you're one of those people who follows that 50% PSU load rule.
Obviously if he's adding parts, that's not enough. However PCPP's estimate is fairly generous, and Corsair's build quality is great, so I highly doubt that he'll come close to the maximum output of that PSU. Remember that 663 watts (or more than 700 overclocked as you say) is the max load on all components at once. When are you ever stressing your CPU and your GPU at 100% at the same time?
Some games, 3d rendering, extreme multitasking with media programs.
You might have a point here, so we kinda should know what the OP plans to do with his rig. If it's just gaming he's going to be fine with a 750W. Unless he's running a 3 monitor setup, I seriously doubt he'll even push both cards to 100%.