We re-powered our 306 with 4.2L F250's a few years back. I believe they are the exact same size as the F300's. We did not re-drill and used the existing holes instead. Our local dealer, Pompano Beach Marine Center (Big Grady dealer) said that they never re-drill for the re-power and always use the old holes. That said, I can speak from personal experience and say that it is never a problem, BUT, if you try and raise one of the engines up out of the water with the other down WITH the engines turned more than 15-20 degrees or so, the bottom inside edge of the cowling will rub...not bad, but it will rub. Since this isnt something you would ever commonly do, it is never a problem for us and wasn't worth re-drilling. Raising both engines up at the same time at any steering angle does not result in rubbing.
I would also say though, that if you did re-drill, you might get slightttlllly better close quarter maneuvering while backing/twisting around. I also notice that I am always making constant slow speed (1300 rpms or less) steering adjustments as the boat doesnt seem to hold a straight line well at those speeds. Not sure if that is a product of engine spacing (I do have counter rotating props), or just a general characteristic of the 306 hull design. On plane, the boat holds a straight line perfectly, in almost any sea condition.
