I've been through this: all of the above is good advice. MUCH simpler, and much safer, to remove the barrel, un-breach it, and drive the old load out the breech end.
WITH, of course, and please forgive me for jumping to an assumption here, you don't personally have experience removing old breech plugs from original barrels, you might want to consider locating and enlisting someone with the bench, vise, tools, and experience to do so without further damaging your old barrel. The process isn't anything like post-grad level gunsmithing, but it's not something to tackle as a first project, either.
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