Personally I would take A30 (as opposed to the A303) with a few stops to warm up with a coffee esp as you will be new to the car. Done plenty of motorways/dual carriageways but flowing A/B roads (or N/D in France) are the best for an M3W.
You may find a spacer on the steering wheel is easier than messing about with the pead box - we both wear 36 inside leg trousers and a 50mm spacer works well. Echo Taffmog comment on narrow shoes. Enjoy the trip!!
