It can be done without damaging the blued finish of the barrels but it is a tricky undertaking though not impossible, but you do need to prepare some specialised tools for the job.
Firstly you need to acquire some feeler gauge blades between 0.025 and 0.035 inch on one side of these you need to fix abrasive wet and dry paper about 800grit on one side of the gauges or just the first 1/2inch from each end, my preferred adhesive is an Epoxy.
To protect the barrel bluing above the rib I do prefer to use Electrical insulation tape because it stands quite a lot of abuse from the abrasives.
Fix the tape to the barrel each side of the rib and push it down well to the joint between the rib and barrel then run a sharp knife down the joint area to remove any tape overlap.
At the muzzle end lift the rib away from the barrels by inserting a match stick, this will make space between the rib and the barrel for you to work with the abrasive you fitted to the feeler gauge blades. By sliding the blades along the gap you can clean the barrel wall and of course the rib, this will be a slow and tedious job and to obtain a better contact you can bend the gauges to the correct profile. Don't stint on this operation the longer you work at it the better the final outcome will be.
Remove the Electrical insulation tape and clean the whole area to remove any tape adhesive residue remove the match stick and push the rib down to its final position because we need to apply a resist to the barrels and rib to stop ant stray solder from sticking where we dont want it to be. Now the traditional method of stopping solder from adhering is to use a soft lead pencil which you rub on to the metal to prevent the solder sticking, so rub this on both sides of the rib and barrel above the joint line.
Next apply an Acid free solder flux to the joint area, I use a flux I make from rosin (Violin Rosin works well but it is rather expensive) dissolved in Ethanol. Then clamp the rib down using a small metal wedge passed under a couple of thorns of wire wrapped around the barrels.
The Solder I use is electrical grade solder 60% tin 40% lead because firstly it works well with a rosin based flux also it has a lower melting point than many other solders. Heat the barrels gently bringing it up to soldering temperature (get it too hot and you will change the barrel bluing colour) and apply the solder to the joint applying heat gently along the joint line to help the solder flow.
This type of repair is not for the beginner but it is good to know how it is done all the same,
So may be this post would be better in DIY Gunsmithing.
OOps I did forget to say that the Solder I use is in the form of a paste and very easy to keep from places you don't want it to be.
