Use the teardown guide for your iMac to familiarise yourself with the layout of everything before you dive in. I was able to get an idea of how much clearance there was once the display is unfixed from the frame, where the optical drive was relative to the base etc
No need to remove the thermal sensor connector from the motherboard - refer to Step 17 to remove the sensor from the optical drive instead
@AliMir - yes, you can reattach the thermal sensor to the new SSD
Just completed the replacement of the optical drive with an SSD using an OWC Data Doubler kit. Attempted to remove the optical drive without disconnecting any cables but found it a bit fiddly to orient the screen for good access. I bit the bullet and disconnected them and found the process less daunting than I imagined. Reconnecting them was similarly straightforward if you're careful.
Tip: you can skip the step for removing the optical drive thermal sensor connector from the motherboard. Still need to remove the sensor from the optical drive but you can leave that hanging and reattach it to the SSD later.
+1 for the straw tip. Worked a treat!