I've used an ATX PSU in the Cerberus X, with Kraken X62 cooler. My PSU was HX1200i. I don't recommend it. I changed my PSU and my AiO later.
Now I'm using SFX SF750 and Corsair H115i Pro.
When you use an ATX PSU with the Kraken, the pump/heatsink top goes against the PSU very tightly. It usually made me feel nervous for some reason, because the ATX HX1200i PSU is relatively longer than many other PSUs. Hence being longer, it was pushing the pipes of the AIO, and clamping them. Besides, I wanted the window on the case to be more useful. And it was, once ATX PSU got replaced with an SFX PSU.
Corsair actually just released SF750 at the time when I bought the ATX HX1200i PSU. I found it a no brainer to not swap it out for the SF750, as it is SFX PSU, Platinum rated and got a very good warranty time.
Anyhow...
My friend is still using an ATX PSU with Kraken X62.
Link:
https://smallformfactor.net/forum/threads/cerberus-x-hackintosh.9942/
I would most likely not recommend 280mm if you want the radiator in the bottom. But you probably know better about your build and what you want in it, when it comes to that.
Actually, I wouldn't even recommend Kraken series of AiO's if using an ATX PSU, going against the pump/heatsink. I love Kraken coolers, I prefer them over Corsair coolers. But having the pressure of an ATX PSU, that might even make vibrations on top, I'd not recommend it. It is very tight-fit.
You could perhaps get an SFX PSU, maybe the Corsair SF series, get front mounting bracket from Sliger and then install an X52 in the bottom. Unless you absolutely want an ATX PSU.
I hope this helps.