You can use the 12mm slipstream fans.
If you look at one of the pictures I linked, the standard 25mm fans are attached to the top of the case, and that's about the height of the RAD. It's a bit hard to tell how much space is left from the angle, but to me, it looks extremely close to the RAM sticks and the top power connector. In the end, I guess it will boil down to the motherboard, but I'm not so sure even 12mm fans would work here.
The 500R case top reservoir has about 29mm of clearance, so sticking even 12mm fans on top of the RAD up there won't work either, unless the OP is willing to leave the top mesh cover off.