On such a long ride it's essential to make sure you have as few issues as possible with contact points, so have a bike-fit or at least a good self-check of your position. Saddle sores, numb hands or back pain are no fun at all. The first time I rode PBP I made the error of having my saddle very straight, so there was too much pressure on my hands, leading to problems with the ulnar nerve, and numb little (and ring) fingers for about 3 months afterwards. The roads on PBP are generally good with far fewer potholes than in UK, but many have a rough finish, so you have to watch the vibrations over 1200km. Gel mitts and bar tape are both worthwhile additions.
A more upright position on the bike may be less aero-dynamic, but could help you avoid neck and shoulder problems later in the ride.
The qualifying rides should give you an idea if whether you are on the right lines. If you suffer any issues on the 400 or 600, then it will only be worse at PBP, so sort it out before you get there.