Use your existing cable and try a few different routings. The routing in the pictures that runs down the gunwale, then across the top of the transom, and into a generous loop down into the steer tube is a very clean routing and keeps the cable safe from exhaust heat. The loop should be fairly generous otherwise it could kink the cable or will cause binding or premature wear. Try to keep the bend radius to a minimum of 12-inches or about a 2-foot diameter loop (about the same as in the box that the new steering will be packaged in). Do not allow any of the cables to come in contact with the exhaust or driveline.
There are only a few available cable lengths; so measure your cable, play with the routing, and note if a shorter length is both better suited to your boat and available. You may be stuck with just one option.
Cheers,
Joe