All displacement, by definition, takes place below the WATERLINE. It would be reasonable to attempt to get the boat to sit close to it's designed waterline with you and your gear in the boat. Remember that this is not going to be an exact calculation because both you and your gear will vary in weight from trip to trip. If you think that the combined weight of paddler and gear is going to be 70 lbs over the boat's designed load capacity, your estimate of 32 liters of additional displacement is about right. The easiest way to add displacement is to lengthen the boat. Adding a form will lengthen it by 12 inches, assuming that's the form spacing. Look at how much volume this adds below the waterline. If it's close to 32 liters you're done. If you need to add more volume you might try adding an inch to the spacing between every form. This works well up to a point. Unless you also lengthen the bow and stern forms, stretching the station spacing will eventually distort the hull shape. I think adding a 1/4 inch around the outside of each form sounds like a lot of work.