The manual shows the location of the hard drive, but doesn't seem to show how to actually get at it. As said, it's under the plastic front panel; there are two tabs located, if memory serves, under the lip in the front. You push those up, then pull forward, away from the computer, and the front panel should swing open and off (there are hooks at the top that it hinges on).
The hard drive is mounted on a plastic sled that you unlock from the chassis by squeezing the wide plastic tab in the middle of the drive's underside (which, since it's vertical, is I think on the right, though I'm not sure). It can be a bit stubborn, but once you've gotten that unhooked it should slide out and you can then unhook the cables and take the screws off to remove the drive from the sled. Swap drives using the same screws, reattach, and go in reverse, and you're good to go.
Some of those units used SCSI hard drives.
Are you sure about that? I'm pretty sure all of the 6500 series used ATA internally, and only had an external SCSI port. The CD drive might have been SCSI, but not the hard drives.
Assuming it's ATA, remember that it'll only recognize smaller drives (I assume the limit is 128GB).
Fond memories of those systems--I bought one back when because the waiting list on Power Computing machines was ridiculous. Still have it, and it actually still gets used occasionally to get stuff off Mac-formatted floppies--the drive in my still works, and I've got a USB card in it, so I can actually get files OFF the thing once I copy them from the floppy.