Yes, welcome to the world of thin metal. Get yourself a copper pipe. Many of them at different lengths. Flatten it with a Hammer. Clamp this flattened pipe to the backside of your welding joint. Make sure it’s tight against the seam. Now weld the seam up. The copper acts like a heat sink keeping heat off your sheet metal and provides a backing where your gaps are too large. Weld in small beads and move around giving your welds time to cool before continuing the seam. Clean clean clean your metal. A lot of times it’s better to open up the good metal farther to be sure it’s thick enough. Both sides should be clean. Your really should be sandblasting these areas. Rust will return. Fancy rust prohibitive paint is just a band aid. I love your tenacity.