I know it sounds funny, but lead can just pour into a void and harden up like...lead. It melts at a minimum 650F so it would need to get pretty hot down there to melt off? Heres the background story: My 35 year old Mazda B2000 had a muffler leak so I bought a new one and attemted to install it. It was a fully welded system so I had to cut and reweld the system. trouble is its near impossible to weld on top of an installed pipe, as well as on paper thin rusty exhaust pipe with a MIG or with slightly better results, a stick welder. So I got it welded back up and there are pinholes everywhere even afgter I ground down the birdshit welds of the MIG and stick welder. so I unbolt it (1 slip fit after the muffler) and try again to fill the pinholes and then blew a huge hole in the thin pipe. So Now I got a .38 sized hole in the side of the pipe at a 90 that I cant easily replace and come up with another idea to slip a larger piece over that patch job and weld that in. Im back to square one with welding that on as Ill blow more holes in it. So my idea was to slip a joint over the pinholes and back fill it with lead, holding the pipe vertical so it flows in. We did that with phone line splices years ago. There is no flexing on this joint. I doubt the pipe gets anywhere close to 650F even a foot after the cat.