I don't think you can use a HD if it requires a adapter. If I remember right, it already has an adapter. I forget what it's called but it basically make it easier to pull out and push in. (Looks like the PCI connectors.)

So there won't be any room for any 50pin conversion adapter.

Found a 18gig 50-pin HD on ebay.

I have a 2gig that came from my PowerMac 6100 if you're interested.