Take the base you have and make sure it fits the bbl surface exactly.
Clamp the base into position so it is square and in position right where you want it,,do this right on the nice blued surface.
Now with a very sharp and thin point scribe, very carefully scribe a neat line in the bbl surface right around the base into the bbl blue.
Also mark a tiny dot or dash mark on the ft or rear face of the base so you'll know which end faces front (or back) when it comes to soldering time.
Unclamp the base. Clean the bottom surface of the base clean and tin it with soft solder. Just a thin coating of clean solder. The easiest I've found to do small parts is a common electric soldering gun. But a torch will do also. Just don't over heat and burn the flux. If you do, you'll have to clean up the surface and re-tin again.
(if the base is crusty from unsoldering it from the bbl to begin with and it probably is,,clean the entire thing up now with a file while it's off the bbl. Either rust blue the part before installation or be satisfied with a cold blue job afterwards.)
On the bbl,,very carefully scrape the bluing off the surface inside the scribe lines you made that define the site base.
Stay within the lines, you're trying to preserve the original bbl blue.
Lightly flux the tinned area of the site base and re-clamp onto the bbl in the exact location and direction (remember that dot or dash line you made).
Before you put the heat to it, take a common pencil and scribble all over the bbl and the site base with it. That'll leave a coating of graphite on the surface. The graphite coating wwon't allow any excess solder that happens to roll out of the joint to stick to the bbl or site base. Plus it'll keep any of the melting flux that runs out from discoloring the bluing.
Now heat the bbl from below w/a propane flame, slowly bring the temp up. Don't overheat nor put the flame directly on the solder joint.
You should be able to see the solder flow. It probably won't need any extra if the fit of the two surfaces is tight,,solder isn't a good filler mat'l anyway.
If you feel the need to add a touch,,have the solder wire ready with the end of it flattened out already with a hammer so it's paper thin. Just touch that thin tip of the wire to the joint and it'll flow a bit right into the joint. Excess will flow right back out. The graphite will protect the blue and fresh steel surfaces from solder clinging to them.
A touch of extra clamp pressure when the solder is still molten is a good idea,,but don't over do it. There should be precious little room to clamp the parts further together. Plus there is always the chance of moving the base off it's mark when adjusting the clamp.
Let it cool on it's own,,no speed cooling with wet cloths ect.
Clean up the area with soap and water afterwards.
