There is no connector used for the backstay. Run the high voltage cable using the same path the old one used. Once you get the wire above the bottom insulator you can use a UV stable ty-wrap to fasten the cable to the backstay. Put the ty-wrap about 6" above the insulator and leave about 10" of cable hanging loose above the ty-wrap. Then loop the wire back down and use a small stainless steel hose clamp to clamp the wire conductor to the back stay above the insulator. The cable where you stripped back the insulation will be pointing down. That way water will not run down inside the insulation and attack the wire. Do not try to seal the end with any goop. In these situations goop just holds in water and makes things worse.
Try to keep the wire 4-6" away from anything metal except for the backstay above the insulator. Also, try to keep it from where people will be tempted to touch it. When the SSB is transmitting you can get shocked or burned from this cable even though it is insulated.
Inspect the connection regularly and if the wire seems corroded then just cut off an inch and reattach. After a few years UV will destroy the outer insulation on the wire and you will repeat this whole process with a new wire. Nothing lasts forever at sea.
1995 Valiant 42