[2002yanks]

A one piece floor would be the easiest. I would put that in first. Brace the crap out of the cab to make sure it doesnt move. cut out the old floor and put in the new one(leave as much of the rockers as you can). Tack it in and check door fitment/gaps. keep tacking and checking until its done. Then i would do one side(doesnt matter drivers or pass). Cut the rocker and cab corner off. see if the pillars need fixing. tack everything in and keep checking fitment then do other side the same.

As far as who to order from, stay away from lmc for body panels(theres alot of threads on here about this). I ordered mine from top banana(they sell key parts). Or nsane hotrods(both vendors on here). I have never ordered from nsane, but have seen some of there work(and they use what they sell).