I use a Propane Torch hand held like a plumber would use.

When things are fitted, tinned and then clamped into place, I place extra clamps on the ribs out from the area that needs reattaching.
That way if the heat starts to melt and flow that solder, it's still in it's orig position and held there. It just cools back down with everything else and all is well.

The amt of heat needed to sweat solder these repairs is not tremendous. Sometimes I do not place extra clamps around and just clamp the lug if it's a small one.

Proper heating for soldering is to heat the metal around the joint, not the joint itself. That avoids burning the flux and overheating things which just causes a poor joint.
Many people get in a hurry and over heat things thinking they have to be especially quick about it or the entire bbl & rib assembly will come apart and fall to pieces on them.
Not true.

If I'm resetting ribs on a bbl that are entirely detached, I will clean the ribs with a scraper of the old solder and flux. Then use a heavy duty elec soldering gun ( I think it's a 200W (?) pistol grip soldering gun) to tin the surfaces.
That goes extremely fast, clean and easy.
I also use the same steel scrapers (like stock inletting scrapers) to clean the bbl surfaces also.
Just home made out of drill rod. Flattened, bent to L shape when heated red. File to shape, Harden and temper and put a file type handle on it
A Flat and a V shape pretty much cover the needs.

With the scraper you can pull it and shave the old solder right off of the surface and a micro shaving of steel at the same time leaving the surface a perfect condition for flux and tinning.
The removed rib usually shows a line where it was soldered to the bbl. With a scraper you can shave the bbl clean right to that old line.
Scrub outside that line with a common soft lead pencil and that steel will not tin.
Then go ahead and tin the scraped clean area. I also do that with the HD soldering gun . It sometimes doesn't quite have the power to tin the bbl's when they get thicker up by the FE lug.
Then I use an old time soldering copper with a sharp point on it heated and tinned with the propane torch to finish up that last little bit and do the Short Rib attachment area.
Regular paste NoCorrode flux from the DIY store and 50/50 Lead-Tin solder.