What is your elevation?
What are your riding temps?
Are you going to keep a full airbox, lid, no lid, snorkel, etc?
With no lid and LRD pipes, at or near sea level with normal warm riding temps, (60 to 80 degrees)
280 is a good start...
And whomever told you a motor would only run on one cylinder if the pistons are in backwards needs to have their head checked...