N.B. The bike bus isn't running this year also be aware that the road at Dounreay will be subject to closure for periods while materials are being moved off site.
That road closure is no issue; the B-road running a little inland is the preferred cycle route.
Good idea to avoid the East coast road, lots more traffic than the official cycle route, which is equally bonnie.
400 miles on the route you are taking should be fine. Leisurely even.