As you haven't had any replies yet, I'll try to give you a few things to think about until others with more knowledge give you better information. You might also want to post your question on the Newport website, as boat-specific information may also be very helpful.
My understanding is that these cables can break because they are routed too tightly around curves on the sheaves, which stresses the wires. So I would suggest taking a careful look at the entire steering/shifter set up to see if the way the wires are routed looks good. Also, check for any looseness, misalignment, worn parts, rough edges, etc.
On my friend's Catalina 30, it required a special order to get the right wire, so I don't think you can just go to your nearest chandlery and get the required parts, unless you are pretty handy and can also improvise a bit.
As I understand it, the installation itself is not overly complicated, unless you need to make changes in the current routing of the wires (see above). There should be enough tension to ensure the wire doesn't slip off the sheaves, but not so tight as too put major stress on it either. It needs to be lubricated with a good engine oil or grease to ensure smooth operation.
If no other advice is forthcoming, you might want to check some of the reference books, like Nigel Calder's Mechanical and Elecrical Manual, which has diagrams of various systems and advice on how they should work.