Hey spike52, welcome to the community! As far as building up your chest and back, im not sure as to what will work best considering you have less than 30 days until Hallows Eve. If you are experienced with clay i would go that route. Psberatta kind of set the standard on COG armor so i would follow his methods if you can. If not why not try my method with layers of foamies. As for the scaling, I ALWAYS do the conversions manually(Use the given dimensions at the bottom of Pepakura Designer, take body measurements, convert to cm or mm, change height in Designer, and done).
Hope that helps. Keep us updated.