You are either going to have to cut the tubes and feed them through a couple of holes before resealing, which would include topping up the coolant and bleeding it,
or cutting a large enough hole for the rad to pass through before mounting,
which do you fancy?
Why do you want to do this anyway. And I doubt bleeding it would work well as you wouldn't be able to replace any coolant you spilt very easily, it would take unknown levels of tediousness, is there space in the case at all?
You'd have to remove the joined parts, leaving a large unsightly hole, I wouldn't go that way personally,
I'd cut a letterbox type hole in the top, (at each end you would need a little more room to allow the tubes through) and mount it so the rad/fans covered up the hole
Something like this, excuse the rushed artwork