Up to you, I would say it will move a little as the floor would add some considerable rigidity to the body. You could just tack it in or take some triangulation measurements in a few specific places and check those before you weld it back together. I never had to repair the amount you are having too, I just had small non specific sections to do. One thing on the replacement of the tophat section, make sure you make some good reference points for this, especially where it changes angle or extends past the body etc. Tack it in place and then stick it back on the chassis to make sure it all lines up.