I initially followed this guide trying to install an unused Mac Pro (trashcan) SSD in my 2012 iMac, but found that the Mac Pro uses the same form factor as the 2013+ Macs.
Instead, I used an adaptor from microsatacables (p/n APL-RET-BLD), a couple sata cables, a sata power splitter, and tandem 2.5 to single 3.5 drive sled to create a fusion drive in my 2012 iMac with a 1tb spinner and a 256gb Samsung 850. As an extraordinary unexpected bonus, Disk Utility recognized this setup and took care of creating the Fusion drive for me. No Terminal tricks required.
microsatacables 2012 SSD slot to SATA adaptor: http://www.microsatacables.com/sata-hdd-...
benchmark info: I did not see an appreciable performance difference between the PCIE adaptor and the built in SATA port. The 850 benchmarked at ~292 write/507 read on the adaptor and 290 write/520 read on the built-in. I'm running it on the adaptor since the cabling paths worked out happier that way