Your cassette deck RCA outputs are line level so they are perfectly matched to the 1/4" inputs on your Audiobox 24c. You shouldn't need to boost or cut the input level. Just keep it at unity gain unless the signal on the tape is a bit hot. Record company released cassettes have a manageable level. Home recorded tapes, mix tapes, live shows, etc. could have a hotter signal so if you have to, you can cut the signal a little on the input channel in S1.
I record stereo mixes into a stereo track, but you can record into two mono tracks if you prefer. Stereo tracks are a little easier to manage, edit, export, etc. You can run a whole side of a cassette into a single stereo track then place slices between the songs.