If your mobo was just lying flat down on a desk you could just lay the 478 h/s fan on top and plug the fan in, think of a testing situation, this would work yeah, there wouldn't be much pressure other than the weight but..?
Far as i can see the top of the cpu just pokes above the 775 bracket it's in so i can't see why the flat base of the 478 h/s wouldn't make a nice clean contact with the top of the cpu?
Your probably thinking why don't you just buy a 775 h/s fan, well i saw an ultra cheap 775 cpu on ebay without h/s now i am very short on cash at the moment but have a 478 h/s and even another few pounds (or bucks) is something i can't afford!! hard times.