Yes, an 80mm fan can be used to cool the HDDs well in your case and that was what I was saying basically. The 3.5" specially designed HDD fan requires quite a bit of room and that depends on your case. The mounting holes usually don't align up and it'll mean you either have to remove a HDD or drill some more holes in
It'll take up a 3.5" shelf only if it fits well though, thats all, and is very good for large HDDs that do heat up a fair amount (which causes failure). They are noisy though due to the high RPM bear in mind and you'll need a 12Vfan connector too to power it.