A hollow under rib is nice if you can find one. I have only used a sold one and cut slots on the underside with a milling machine. By necessity I leave a web of steel every 5" or so.
Flux? Acid flux is fine to tin the parts. Once tinned I wash the flux off. I then re flux with rosin. You can by "liquid rosin flux" from electronics supply stores. You could make it with powdered rosin and alcohol. Rosin is not corrosive. IF you use acid paste flux there is a chance you rib will separate after a long period of time.
Heat, I use a MAPP gas torch and or a gasoline plumbers torch. I start at one end and move the heat down as I go to tin the barrel and rib. Use the minimum heat necessary. Tin with a steel wool ball, flux and solder. Rub it in. I make no attempt to heat it all at once. One tinned washed and rosin fluxed, I use a steel ramrod substitute to align everything. I do the pipes and rib in one shot. I use hay wire and wedges. IN a addition I use C-claps to make sure the joint is tight.
Once I am happy with how everything is aligned I heat it . I start a the muzzle and work back. I heat from the underside, The barrel side. IF there're is any doubt as to hot enough I tap the rib with a piece of solder and see if it melts. The the barrel turns blue that is too hot. Again I work my way down the barrel, not need to heat it all at once. If I get no solder squeeze out in a spot I add solder with rosin core solder. It will wick into the joint.
The whole thing is easier than it sounds.