I'd give Boston a 92% chance of winning this game.
That's truly and totally unrealistic for any MLB-game.
A winning expectancy of 92% would imply fair odds of -/+1150 (excluding additional juice). Well, hell would freeze over before we'd see such a line on a baseball-game.
For what it is worth, -320 implies 76.2%. I presume if you think they have a chance of 80% to win, the BoSox are indeed the side to take.