Bill,Yes the GN and BN car will fit the early 70s as will any road that was part of the BN merger.
I recall seeing PRR,NYC and Reading 50' boxcars as late as 83.
Railroads didn't rush into repainting cars and in some cases locomotives after a merger.Chessie had lots of locomotives still in C&O and B&O paint scheme well into the mid to late 80s..