I lived in Liverpool in the' early 70s and had no problem as a United fan, it was just a normal rivalry. By the late 70s it was changing to hatred, fortunately I'd moved by then. The heart of it was our resentment of Liverpool's success, and Liverpool's fury at our insistence that we were the bigger club. 'but we've won blah blah', yeah, but we're the bigger club. Come Fergie and it was their turn to resent us for our success. We were still the bigger club of course.
The ship canal is historically true but not relevant. The American civil war was another one, Manchester supported the North, Liverpool the South, with Liverpool paying reparations at the end of it. That wouldn't be relevant to the 70s/80s either, which was football.