No reason you can't gas weld a replacement quarter panel on the car. Gas welding was the go to process for sheet metal replacement for the first 70 years of auto body work. Only the economics of commercial body repair led to it no longer being monetarily feasible due to it's a slower process and it required a more skilled employee.
Do some interweb searching on the subject and you will find lots of information. There are some very high quality welding torches still available today to make the job easier. Plus you can purchase one brand new and the money saved over a mig or tig will pay for your new panel. And no matter which way you go practice and attention to detail will be the keys to a successful completion.
And remember it's just sheet metal if you make a mistake just step back take a breath and start again. After all you started out replacing bad sheet metal.
https://www.tinmantech.com/products/welding/
http://www.spiuserforum.com/index.ph...talworking.14/
Good luck