CharlieS
GOLD Star
For a part that is less visible, I wouldn't rush out and spend a pile of cash on a gas welder. But you'll get much better results if you do.Flux core will be ok if you just make tack welds on top of each other and let cool every so often. If you can reach the back of the panel put some copper or aluminum bar backing the weld to absorb heat. I seen my old roommate replace his quarter panel on a 68 Camaro and ran a almost continuous bead with flux core and the whole welded area of the panel sunk in and looked like the inside of a boat hull.
Another thing I like to do is keep an air hose with a nozzle handy. You will catch stuff on fire (undercoating, for example), and it allows you to cool welds.
I am not an expert welder, but I have had some luck with doing this kind of replacement with tack welds, spread out, moving to different areas, eventually filling all gaps.