My experience was a little different than a lot of guys. I preferred heavier shafts to help keep it all on plane. I also liked the really oversized grips, it felt more natural. The other part, you probably want to be in the XX or XXX flex range for that length, you can tip an X about an inch and it will be "good enough", but it's not the same experience as a dedicated LD shaft.
In terms of swing, make sure you keep your rotation, that's the biggest key to everything. It forces you to go back slower, the arc get longer and then when you unwind, you get the benefits of that extra length. With a true LD club, the distance difference in my experience has been way greater than the 5 yards per inch of shaft rule. I have seen jumps of 30-50 yards depending on the strike of course. I'm right around 320 (still around 300 when I'm not going crazy hard) with a really aggressive swing at 45", when I was doing LD it was the same, but I got a few over 400 and consistantly got around 360. The launch, spin, landing etc are all different, so you won't see the traditional linear progression of distance if you get it dialed in. You obviously have good club speed to start, so you should see a pretty big jump.