Naqvi Sb. it is frustrating that simple commonly available tools and repair facilities available in other countries are just a huge issue here. I may even go as far as saying that what is possible in Karachi is totally foreign in Pindi/Islamabad. After getting that out of my system i can proceed with the reminder of this post.
After seeing the size of torch and soldering iron with the radiator wallas around the area, I believe that the heat from the propane torch will melt the solder holding the copper interlacing fins between the adjacent tubes. I am afraid I will land myself in more trouble than I am already in. I will share some pictures below showing the radiator construction and the home repair.

An old picture. It was taken while doing up the engine area. It shows the top water tank of the radiator

Steel frame surrounding the radiator core

Small window cut to expose the leaking tube

Final look - fabricated fins fitted in the circled area
Of course as you said, the real test will be to see of the repeated expansion/contraction cycles hold or not