Sell your barrel and buy a new or used barrel. Or swap. There are always barrels for sale.
Cutting the barrel will make it cylinder bore. Cylinder bore is the biggest pattern choke. In most shotgun stages, cyl bore will have trouble knocking down plates with bird shot. improved cyl or modified are preferred. They are good for plates with birdshot and will shoot slugs just fine. After cutting the barrel, if you want to thread it, it will cost $100 plus. You might as well shoot what you got, until you find another barrel.
You will want at least an 8 round mag tube. That is the most allowed in some divisions. Less and you will be doing a lot of reloading. I have a 8+1 extension and it ends about an inch beyond my 21 inch barrel. So i would not go shorter than 21" and not longer than 24". Easier to swing around barricades and such.