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My ribs are all hollow ground and this is the method I use to solder them down.
Here in a nutshell is what I do when fitting a top rib. I use my bench grinder with about a 6 inch wheel on it to finish taper out the middle section, the part that is hollow ground. You want the flat bottom of the rib to be at about 45 degrees to the wheel as well as to the grinder support. You can do the quarter rib and the front sight section the same way for a bit, but as it starts sinking down between the barrels, I stop using the grindstone and switch over to small grinding stones in my Foredom grinder that match the diameter of the barrels at those locations. You will want to work slowly and try to get it to fit both barrels as well as possible keeping the rib centered at all times.
Once I have it down where it belongs, I mark along both sides with a pencil, I do the same on the bottom rib which should be fitted the same way. Then I tin inside those lines so that all the unexposed areas are covered in solder between the barrels, this helps to prevent rusting. I also tin the areas of the ribs where they will meet the barrels and the whole bottom of them. Since I use Brownells Hi Force 44 to do the soldering of my ribs, this makes the barrel assembly much stronger. All the areas soldered and the complete barrels need to be washed off in HOT water and I use Dawn dishwashing liquid soap in it to help cut the corrosive flux off the whole assembly.
After washing I rinse them in HOT water for awhile so that they get good and hot, then I dry them off using paper towels and run some cleaning patches down each barrel, the last ones with a little oil on them to protect the bores.
When you are ready to begin soldering them together you need to re-flux everything with rosin paste flux because it is non corrosive. You can find it in some Radio Shack stores or order it online. It is made for soldering small electrical parts together. You will want all your tools handy once you start to solder. I like to place modified C clamps across the barrels side to side at each spacer, that keeps the regulation set while soldering on the ribs.
I usually solder on the top rib first, after clamping it up using two 3/8 inch steel rods, one in the hollow ground section of the top rib and one full length one from the forearm bracket forward to the muzzles on the bottom. Here again, I use my modified C clamps, each one has both pads hollow ground to roughly match the round contours of the barrels or the rods. You want to use the smallest size you can get by with, most of mine are either 2 or 2 1/2 inch clamps. I usually have a couple handy that have only one side ground, that way I can put the flat pad on the flat top of the ribs at each end. You may also need a third one done this way for clamping the quarter rib behind the forearm hanger.
I try and remember to put some flat polished steel pads under the flat pads so as not to score up the flat top of the rib. I start soldering at the heaviest end, the quarter rib, and I make sure it is down completely before I move on to the middle section. I do the front end last, the reason for doing it this way is because the rib will expand lengthways as you solder it and you only want to get it hot enough to make the solder flow. When you finish up there will be some dark burnt flux to remove, I just use steel wool and gasoline to get rid of most of it.
I also have a couple of other items that I find work very well, one is a Plexiglas cutter. It consists of a long handled thin flat blade that terminates up front with a pointed end, and slightly behind that is a sharpened hook that is used on the pull stroke to score the panels of Plexiglas. It is the perfect tool to remove excess solder from between the barrels and ribs. The last item is a large cheap wooden handled screwdriver that has been sharpened to a pointed cross section, leaving the blade flat but sharply sloped to a point. I have found this to be the perfect soldering iron for getting the solder to flow in between the barrels and the ribs. If you find a stubborn area that is not wanting to go down, just apply some flux to the tip, run on some solder, and rub it up and down the joint. This lets in some extra newly fluxed solder and will help bring the joint together. I hope this helps you. Bob