Get rid of the wire all together and replace the entire length of the halyard with 8-10mm spectra (dyneema covered with polyester). Attach the shackle with a halyard knot and my preference on the cockpit end is to remove about 300mm of core, and then tuck the cover back into itself leaving a loop with a few locking stitches. This leaves a small loop to attach to to help pull the new halyard through.

Rope to wire splices are a pain and there really is no point any more.

Jode5 said..
Not sure what you are trying to do. You are not trying to just remove the wire and joining the Dyneema to the existing rope part of the halyard. If this is the case, it is not the way it is done. You replace the complete wire rope halyard with one length of Dyneema. (No joints)

Thanks for comments.

I did replace the entire main halyard with Spectra to try it. It runs through sheaves fine and doesn't clank on the mast in a breeze and is much lighter and nicer to handle - on the sail side. But it got worn quickly compared to the wire.

But the jammers can't grab it because its too thin and its a pain to haul by hand. Thats why I aimed at a "plastic" halyard attached to a 12mm "rope" sheet.

I got the tub re-rigged recently and my old 10mm double braid halyards - which were steel to double braid when l bought the tub before replacing them, few months after - got replaced by black spectra 8mm lines.
My clutches handled the new, thinner lines amicably. No slipping.
Y black? Because they are less sensitive to sunlight according to the knowledgeable.
The jib halyards were getting the same treatment in red. Still 8mm.

I found the new arrangements very good, no complaints.

Wire halyards have no place on a see self respecting modern boat, methinks.