Welcome to the board. Lots of great help here.
You'll discover that for the most part...the burbs and PU's are the same from the doors (cab) forward. Even the engine bay is basically the same. Most of the differences are behind the driver
I've not installed it yet as my project has been on hold for a couple years but all the research I did at the time said that
http://www.steelerubber.com/ has some of the highest quality weather seals.
Most of the time, especially for body panels you'd be better off searching for NOS or to refurbish parts off donor trucks, junkyards, ebay, or craigslist. IMO. I have heard good things about top banana when it comes to repop body panels.
However, sometimes you do have to buy new and places like
www.truckandcarshop.com http://www.lmctruck.com/ http://www.brotherstrucks.com/
do have serviceable selections at least, but most of the parts that are repop'd for this series of trucks are chinese imports and on the lower end of quality.