it's a truck, gm didn't put too much thought into fit and finish on trucks back then.
my doors fit like poo poo, you can massage the door or opening edges to get the fit better too.
you have to remember that your looking at your door gaps from up close
unless they're real bad, get them close and call it a day.
stuff like that will drive you nuts
like martinsr said, spot welds around the gutter to take the old one off,
a little more work to get the usable one off though
go here http://67-72chevytrucks.com/55-59chevytrucks
a lot of guys over there are replacing the roof skin
you can find build pics of the process