I had both a data drive and my parity drive fail at the same time (bad PSU). This leaves me with a cache drive and two additional data drives in the array.

 

I know that the data on the 8tb data drive is gone. My question is how, using New Config, do I ensure that I retain the data on the two remaining drives? What do I need to select on the "Preserve Current Assignments" to allow me to spin up the array, add a new Parity drive, add a new data drive, and retain the data on the existing drives?

After using New Config then when you start the array Unraid will recognise any data drives are in Unraid format; leave their data intact; and build new parity based on the new drive set when you start the array.