Probably a better idea would be to use the second FDD bay* instead of the 5.25" bay.. the HDD would be closer to the fan but separate from the present 3 HDDs so heat would be less of a problem. The 5.25" bay would be a good location if there are no plastic HDD retainers involved which would not be as efficient heat conductors as direct contact to case metal.
Read extensive comments on the subject herehttp://www.tomshardware.com/forum/227370-32-hard-drive-heat-issue
*that's if the mounting screwholes match. HDDs to FDDs do match but there may be retainers/adapters that wont.