Hi
It might be possible to move the fan upwards as Gloup_Gloup suggested or to even move the fan to the opposite side of the fins so that it acts like a puller rather than a pusher, the effect on performance would be negligable.
If you cannot get around the problem in this way though I don't see why you shouldn't do what you want to but you would have the same chance of failure/success as any time when you add more memory to an existing kit.