59 f100 lower door skin

hello
i am having a lot of problems replacing my lower door skin, i am on the third try it is a two foot section as soon as i weld the panels together and crimp the edges the panel flexes in, and it is not easy on these 59s to work on the inside is there a trick to this. i am useing 20 gage. this is the only thing holding me up to complete this project. thank you:)