I have a feeling that there is more to your problem than coming up with more length and/or depth.
From an engineering standpoint no hull is designed to be loaded to 100% of capacity 100% of the time. When the paddler contributes that 100% you have a point load that was not on the design sheet either. Getting in and getting out can put those point loads on very small areas.
If you were building an old fashioned glue and screw boat you could increase the component sizes and/or the overall dimensions to take up the load. With S&G there is not that option.
The project could fail if his weight overloads the deck when he gets in and out. If the boat rides too low it may not handle well. And on and on.
Rather than trying to build on guesswork I would want to see this big boy in numerous plastic boats and I would try to duplicate the one that works best.

: I'm about to start building a kayak for a rather large guy. I figure around
: four hundred pounds. I was planning on increasing the sheer of a Mill
: Creek 13, and wanted to know if someone has a way to calculate how much I
: would need to add to give it another 100 pounds of capacity.