Read my to comments above before following this:
I still think you'll be just as happy with a SSHD and its a lot less work. Here's a link to Seagates web site that explains what it is: Seagate Desktop SSHD and here's the spec sheet: Desktop SSHD specsheet. This drive supports the newer '11 onward internal thermal sensor the iMacs have so its a direct drop in for your current HD. If you use anything else you'll likely need an adapter cable. Here's a link explaining OWC Updates Hard Drive DIY Kit For Expanding Factory Installed Storage Capacity for All 2011 iMac and 27″ 2012-Current iMac Models.
Follow this IFIXIT guide: iMac Intel 27" EMC 2639 Hard Drive Replacement on the needed steps.
OK, you're going for it! Apple blade SSD it is!
Follow this IFIXIT guide: iMac Intel 27" EMC 2639 SSD Replacement.
But before you do review the steps between the HD Vs SSD as you'll be removing the logic board to get to the SSD mount Vs the HD swap out. Are you up to this? do you have the needed ESD mat and wrist strap and its correctly grounded to protect the logic board (remember its winter so the air is dry - lots of static electricity!!)
Don't forget to order a set of adhesive strips before going either way!
I wish you luck which ever direction you go! Please let us know how it goes.