My Contender (2005 25 Open) came with a Furuno 585 & B260 525T-HDD. I am looking to replace the

This wire is for Mix and match transducers only. If your B260 is a mix and match it will have a similar short extension on it for your furuno. You could then swap that cable with this one. It sounds like you may not have a mix and match so I would recommend using the Garmin wire block adapter. Here is the link- https://www.boemarine.com/garmin-bare-wire-transducer-to-12-pin-sounder-wire-block-adapter.html