I think it's been that way since the beginning...in the 30's they put the radiator shell on the chassis first, then the front fenders with the running boards and then the body.
Check out this 1946 assembly line film, it's a treasure trove of details. You'll see the front clip getting installed already painted and trimmed out!
I know this is cars, but the trucks came down the same line and would in theory be assembled the same way, check out this picture of a truck cab behind a '57 four door sedan, on the line!
