Some info in case you are still at it: If you have a solder tool, you can simply replace the HDD with any SSD if you solder the pin used for Apple's temperature measurement. Many have done this. Apple does the equivalent (by using a jumper connector on the logic board) on the SSD-only 2011 iMacs, it's the same solution.
As the SSD does not get hot anyway (unlike the spinning HDD needing 12V, too), there is no downside. The result of bridging the temp sensing to 0V is only that the HDD fan will spin around silent 1000RPM and not more.
View attachment 930207
Apple solution
DIY solution: bridge pin 11 to 10 or 12 (either one, on either side of the connector):
View attachment 930208
View attachment 930209
or
View attachment 930210
If you don't want to solder but get the same result, use a SATA Power cable splitter like this; they already have 10, 11, 12 bridged.
View attachment 930215
Next, since you are at it, add USB 3 into your 2011 iMac:
Hello folks, I have been following the thread about graphics card upgrade 2011 iMac Graphics Card Upgrade for some time, and someone mentioned that he has upgraded to USB3. So I started my own USB3 project, and succeeded, at a cost of around US$ 20, plus around 3 hours work. I replaced the...
forums.macrumors.com
View attachment 930211
View attachment 930212View attachment 930213
Enjoy the speed.