I currently have a 6GB HD from an old iMac in my 6500 as well as 1GB and 3GB SCSI drives. The 1GB is mounted upside down in the top 5.25" bay ( I put some screw holes in the top of the bay) and the 3GB is mounted on a custom rack of sorts attached to the rail supports on the tray with the DB-15 connector where that coax card or w/e would go if it was a higher end model.

I taped the rails to the side of the 5.25" bays in case I needed them in the future.

I can get pictures if anyone wants.

Oh, I also slapped an 80mm fan below the 3GB HD.