Just cyl choke(s) are good. Use spreeder loads in them if you want even more.
As far as filling the void at the muzzle on cut bbls, don't be concerned that the entire assembly will fall to pieces because you apply heat to solder it up.
I routinely solder and re-solder the muzzle ends closed after rust bluing (I open them to allow the bbl's to drain better if I can).
Clean the edges of the bbls and opposing ribs down in there, scrape clean. A tiny amt of paste flux, insert or reinsert the keels if you have any.
If you don't, a small tuft of steel wool pushed into the opening a ways will dam it closed so the solder won't drip down the opening freely. It'll absorb the flux with the first heat and then the solder and become one mass soldered to the bbls and ribs in there.
For heat I use nothing but a pistol grip electric soldering gun (Weller). Plenty of heat( I tin the ribs and most all of the bbl surfacees with the same gun when relaying ribs), common 50/50 or 60/40 lead-tin solder.
No wet wraps or clamps, it takes but a few seconds to heat and flow the solder and be done. Nothing comes undone.
File trim it flat and polish smooth.
If there are any voids in it, it was dirty. Clear them, scrape them clean, reflux the areas and touch up with the soldering gun again and retrim.
One of the old school plug in soldering irons w/the large copper tip works well for this too. Tin the tip well to start.
It'll have plenty of heat for a quick solder job on the muzzle.
Stay away from a propane torch IMO,,no need for one to do the muzzle fill-in job. You can get into trouble using one of these unless very careful and apply too much heat w/o noticing it. Then you can start to loosen ribs or overheat the steel and have problems.
