I would think it's easiest to add length in the middle of the boat. You could put two shortend spaces between forms where normally one space would go (e.g. two 18 inch spaces in place of one 24 inch space. This could be done by duplicating the form close to midships, thus using the same form twice.

To get a volume, if you're mathematically challenged, make a form, bend a piece (or pieces) of doorskin around it (may need two forms). Basically, you make a mockup of the center or the boat. Stand it up like a trash barrel. Fill it with a known quantity of water, and measure the depth of water. Now you know liters per inch at midships.