My dad's old iMac was getting slow. My dad is 78 and he likes to browse the internet in style. So I surprised him with an upgrade of his old machine.
The repair took 3 hours and it went really well due to the guide on this website. Also the old iMac is not made for 2,5" drives. There is no way to fix them in the casing without a 3,5" adapter bracket.
If you are daring, impatient and generally optimistic you can simply chose not to care and dump the ssd into the casing without any fixing. This is what I did and it works just fine.
Get a friend to help you. Preferably someone with small hands. You need small hands (like those Foxconn workers) in order to connect and reconnect the power cables of the display.