Brandon, It turns out that there is a Chesapeake 18 hi volume(!) on file in case you are interested.

Seems to me the decision isn't so much a "kayak" for nearly 500lbs of displacement but what use that nearly 500 lbs displacement kayak is intended for. Flat flowing rivers doesn't need an efficient long skinny hull or a very high volume sea kayak with a beam narrow enough to roll may be more responsiveness than you may want with 260 lbs rolling back at you. I don't have experience with the loads you are describing but the folks on this board must have some good pointers for strengthening the hull, us overbuilders would have a field day with this one.