Isn't the Calumet 540 the square rail model made by Cambo? The parts are interchangeable with the square rail Cambo models and the bellow and backs are interchangeable with all Cambo models. I believe the round rail model is the Calumet CC400. If it's the square rail model I would keep it,you can find rails from 12 inches to 30 inches and bellows from bag bellows to extra long bellows.
It's a very versatile camera.