I wouldn't put any more heat into it, you may make it worse or pull it into a diamond shape, it just needs a bit of persuasion, on a flat floor support 2 opposite corners on blocks, a couple of offcuts of box section are ideal, then hold one of the other corners on the floor, stick the wheel of your car on it, then stand / bounce on the high corner to press it down, If it wont go then you either go and find a fat bloke to help with the weight or run a cutting disk down a joint just enough to allow it to move and re weld it,
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Yep, that's exactly how we do it!