get a new backstay....
...that is the simplest and cheapest solution. Any kind of adjuster rig will cost a couple hundred dollars, and you'll still have to cut the backstay to make it fit. If you want to set it and forget it, then replace (or shorten) your existing wire. Maybe put a longer turnbuckle on it. Before I made any big changes I think I would try re tuning the mast first. Tuning instructions are in the Catalina 30 owner's manual, available at the C30 owner's website. After that you might make another assessment of the problem. Your lee helm could be caused by other elements such as sail trim or condition. Good luck.