Capn,
Not sure what your height & weight are. I'm 6'5" and 245lbs. Prowse was two inches taller than me and the same weight, but certainly far more buffed than myself when he played the roll. He was a champion weight lifter and body builder. Me, I'm just a tall large actor. I was a little concerned the first time Lucasfilm had me put on the original costume. I was not certain I would fill it out enough in the chest and shoulders when I initially saw the suit (that's a funny tale in and of itself). It turns out the costume was a bit more forgiving than I expected. For that first official appearance I did for Lucasfilm as Vader Don Bies' wife Anna padded my chest just a bit with a folded towel we had on hand. Later she created a specially made padded T-shirt for me to wear. Eventually I built my own chest muscles up enough that I didn't require any padding at all.