My Progress on a 1968 Plymouth Valiant 100

So I was going to replace the transmission filter and fluid in my 1968 Plymouth Valiant 100 when i noticed the frame in a section was pretty rusted on the drivers side. I am planning on fixing it with a welder but im pretty much a total newbie to welding (i was already planning on welding it anyway there are sections of the floor with holes in them) any tips or information would be greatly appreciated i don't plan on giving up on this car and i will continue to work on it in the meantime. Thanks in advance

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