Internal use of 3.5" disks is not going to be possible on the HP ProLiant DL580 G4 system.
Given than that the system is an older generation ProLiant, the best matching solution (and path of least resistance) is to obtain a period-correct external enclosure for the system. In this case, that would be an HP StorageWorks MSA60 12-bay enclosure. Pair that with a Smart Array P800 RAID controller and you'll have a workable solution. You will need 3.5" drive caddies if you don't already have them (HP part #373211-002)
The MSA60 and P800 can be found cheaply on eBay.
I'm decommissioning a DL580 G4 as I write this. It may not be worth investing any money in that hardware. It's been eclipsed by two model revisions.
A cleaner solution, if you simply need the large form-factor drive bays, would be to find a ProLiant DL180 G6 unit. It can accommodate 14 LFF disks and would be sub $3000US.