While Boeing knew they all had similar fuselage sections the model manufacturers never seemed to grasp that fact. You'd be amazed at the number of different sections they've managed to produce. I tried to do something like that years ago using a couple of Frog 707s and an Airfix 727-100 but would they fit? NO way! I still have the wreckage somewhere, maybe I should dig it out and try again.
Actually now I think about it, they may have been two Airfix 727s and a singular Frog 707 but you get the idea.