: can I
: take a kayak plan and open up the cockpit

That's what I'll be doing on the tandem kayak I'm building now.

: My main criteria are in order of preference: stability (initial and final),
: room to carry a day pack for two adults, total capacity of at least 400
: lbs, as short as possible, tracking, low in the water so wind doesn't
: significantly affect the boat, speed.

The Mill Creek 16.5 will be about the shortest double you'll find. If you like strippers then you could do the deck in cedar.