They make a plug-in for the Z-IV slot. There's no other way around it. Once you do it, you'll be glad you did. I didn't like the fact that you must permanently remove the RF shielding, though, nor the fact that you could never use the HD cradle, as this add-on interferes with both items. I've got a thick piece of cardboard on the bottoms of my 2 2.5" HDs wrapped around with electrical tape so that it won't short out either the A1200 or the HD. My A1200 is still in the original desktop shell.
Given the size that your HD is that you descibe in your first posting, I'm 100% sure that it's at least a PIO-5, if not a PIO-6 type.
If you feel nervous about doing it yourself, I can come up there sometime on a Sunday & perform the surgery myself, for free. I worked up in Madison last year & was commuting 2½ hours each way every work day for the first month of the contract. Whereabouts in the Badger do you live? You can write me at my e-mail account listed in my profile, if you want to.