Peel back the exterior sheath a couple inches and snip off the exposed lead core. Use a nail knot or a five turn Uni- Knot (what I prefer), leader only. Pull tight, apply some instant glue and trim the nylon and leader tag ends.
You could shorten the leader to six to 9 inches and tie on a micro swivel which gives you protection against any twist and makes changing leader
length and strength easy without having to re-attach to the leadcore.
What pound test leadcore did you purchase. Eighteen pound test has the same core as higher test as only the sheath determines the strength.