The only mention you made to length was the Chesapeake 18LT. Given your height and weight, from what I now and have read, an 18footer for you would not be a good kayak to build. Too much bouyancy. Something in the 16 - 17 foot range might be a better choice.

As for the cockpit length, you can make it as long as you like, it is just a matter of bulkhead/ footpeg placement.

Another item that may influence your choice is whether or not you want to build from plans or a kit. From plans it can cost $250- $400 from kits $900 - $1500.