Hi Ian,
I retro-fitted a chart plotter onto the tubes on my 325. If your tube connections are similar – everything is hollow and you can get a SeaTalkng cable in – however I had to elongate the hole at the top very slightly to get the connector plug out!
There is a picture of this on my blog here.
I suspect your tubes are straighter than mine so you may find it easier.
Regards,
Mark
Molia