I recommend, before you order anything, to check under the floor where the cab mounts are. if that part is rusted then the under floor parts, which weld to that, are likely gonna need some help too. the step is also connected in the bottom of the hinge pillar as well as the lower fender mount. there are a few other parts you may need. inner cowl needs to come off if doing the whole panel, for one.
here is a link to the assembly manual showing how it all goes together
https://www.trifive.com/d1/55-59Assy.pdf
sheet 6 and on
do some good precise measurements and X brace the crap outta the cab if you plan on replacing that part. pics with measurement locations work well. when assembling tack stuff together then install the door so you know it fits before final welding. actually, make sure the door fits before you start so you have a baseline. sometimes those replacement panels have the wrong threads for the door hinges so check before installing fasteners. like same size but course instaed of fine threads.