You pretty much need floors in place before anything else, especially if (when?) you find that you need to replace intermediate or inner sills (rocker panels) as well as outer ones. Probably the best advice I can offer is to do one side at a time, keep the tub on the chassis if at all possible (to use as an alignment "jig"), and be prepared to brace the door opening...or leave the door in place.
Oh, and check door and panel alignment early and often (kinda like the classic "This Old House" adage of "measure twice; cut once": you only really get one chance once you've started the actual welding).