Trying to get ahead for 121 training is not required, and is not likely to be helpful since every airline has different procedures and and a different version of FAR 121 by which they operate.

Preparing for the interview is what you should be concerned with...

Know basic 121 rules (get the internet gouge to narrow it down).

If you expect a sim eval on an interview you will need to prep for that using (preferably) an identical of similar sim. If that's not practical, use MS Flight sim with an aircraft similar to the sim you expect to use. Any other high-end PC flight sim game would probably work too, just use an appropriate aircraft model.

Other than that, be knowledgeable about your current job and aircraft.