I was thinking about it more and even if I could fit single rads it'll prob be only 120mm ones. The fan at the back will only allow 120mm anyway. And, as you say, the size of the rad is greater than the size description so 140mm's are a no no.
It's a shame as I'm running 4 extra 140mm Be Quiet Silent Wing fans and was hoping to mount these to rads.
I'll check out your links for the external mounts though, thanks. Despite the fact the Fractal Design R3 has water cooling tube holes out the back of the case, they've made it very difficult to actually fit water cooling! lol