@MacMiga
Sorry, my fault I did misread the original post.
Now I understand the question (I hope) I do have another point...
IDE cables are only meant to be used internally. Not that there`s anything against hanging them outta the back and into an "expansion tower".
For a start, they aren`t shielded, so you could get noise/interference into the data signals.
Also, AFAIK the maximum recommended length of and IDE cable is about 30 cm. Using a buffered interface wont help extend the useable length, but it will save your CPU if something goes wrong.
Your idea of sitting the 1200 ontop of the AT case is a good idea, cos that`ll minimize cable length.
Also, you get the ability to power the 1200 from the AT PSU and get rid of the stupid CBM power brick.
Personally I`d use SCSI in this situation, since it`s faster and can take more devices.
So, yes, it`s a perfectly possible project. AFAIK there were even commercially available case conversions that just held the drives and supplied power.