I replaced the (long-time broken) Optical Drive with an SSD, using a mounting from a previous upgrade. The total time was not very long, but I had to interrupt progress fron time-to-time. The Repair Guide mentions to tape back the loose cables before re-inserting the logic board -- OH YES! It was frustrating to do it twice, but no harm done.
Followed the Guide (almost) to the letter. Now I boot the iMac from SSD and made a symlink to the HDD /Users node, now I can use the whole hard drive for /Users data.
When the iMac is opened, it's easy to clean out the dust and cat hairs from the insides.