The difference in temps is normal. AMD overdrive is monitoring the internal sensor inside the chip. Asus AI is monitoring the sensor on the socket located underneath the cpu (farther away from the cooler) so this is normal to have the socket temp higher than the cpu.
But as for your question, I mounted a wc loop for a friend in a similar case (thermaltake dokker), used the EX 240 rad, and had to mount the fans on top of the case with the radiator inside.
Ex 240 kit can be found here: http://dazmode.com/store/index.php?main_page=index&cPath=178_219
Looks like the overseer may have a tiny bit more room, the fans may fit inside. The RX radiator would still be an extremely tight fit.
Optional cooling: CM hyper 212 EVO.