Best place for additional 1tb ssd

Sorry guy not so simple!

You have two different drive interfaces in this system a SATA I (1.5 Gb/s) for the HD and a PATA (80 Mb/s) which is a huge difference is throughput! Just from this the better interface for your SSD would be the SATA HD port. But... There's more!

Your current HD is a 3.5" drive unlike the SSD which is a 2.5" drive and the optical drive carrier to hold your second drive. So you have a physical size issue so you can't move your current drive over. Then there is the issue of the current HD frame needing an an adapter frame to support the smaller 2.5" SSD. But... There's more!

Now we have the SATA I/O speed issue. Most current SSD drives are Fixed SATA II (3.0 Gb/s) or SATA III (6.0 Gb/s). Remember at the top here I stated your system was only SATA I. These fixed SATA drives just won't work in your system! At least the Samsung 850 EVO you have selected will work here as it has SATA port auto sense. But, you'll will need to make sure the SSD's firmware is up to date.

But how about doing this instead!

Get a Hybrid SSHD drive to replace your current HD. With it you get the performance you are looking for with the deep SSD cache it has and yet get the deep storage of a traditional HD. As you are using the faster port there is no lag and it's cheaper!

Here's the drive you'll need: Seagate Desktop SSHD. If you review the specs sheet here: Seagate Desktop SSD Specs. Take note on the line SATA Transfer Rates Supported (Gb/s) here you'll see all three SATA data rates are listed. As its a 3.5" drive it will fit the HD frame without any issues.

Here's the IFIXIT guide you'll need to follow: iMac Intel 24" EMC 2134 and 2211 Hard Drive Replacement