If it's parallel octagon, or straight tapered, I'd cut off the breech and leave the muzzle alone, in case the barrel has run out, which would have been left at the breech when the plug was installed.
If it is tapered and flared (swamped) I'd take two inches off each end, and rethread for the plug. But I have a lathe and can do that easily. It is still possible without a lathe, but more time consuming and not as easy as with a lathe.
You see that there is more than one way to approach this.