My old hhd was failing, so instead of just replacing it, I decided to upgrade to a fusion drive.
For best results with a fusion drive, the hhd should go in the optical drive enclosure and the ssd should go where the original hdd was. This meant using a 2.5" to 3.5" SATA adapter to hold the drive in place.
I followed the advice of some recent fusion drive upgrades of preserving the recovery partition on the ssd (google fusion drive with recovery partition for more info) and everything appears to be working great. I haven't tested the recovery partition yet, so I'm not sure that it will show up during boot. Also, I managed to get both the hdd thermal sensor and the optical drive sensor to reach to the new hdd in the optical bay. It seems to be working great and the fans run very infrequently. If you attach the sensor to the ssd, I've read that it will read out of range on the cool side and cause the system to assume the sensor is faulty and run the fans continually.