Adding a 120mm Fan to an ATI 4890?

Hi all, I have 2 4890's in my pc that I'm trying to get a few more months out of (until the GTX 670's come down in price) but the fans are now noisy and driving me nuts.

I've tried the usual cleaning etc but still noisy so I wondered about taking the stock fans off the heatsinks and just attaching a 120mm fan to each heatsink?

As I will replace them soon its not worth buying new heatsinks and I cant get replacement fans. I have a couple of 120mm Akasa Apache's kicking about, do you think this would work ok?

Cheers.