Just kidding.
In the first book by Kulczicki (The Kayak Shop) there were plans for the Yare which is the sister craft to the Severn. Yare is 16'2" to Severn's 14'6". Yare is 24" wide Severn is 26". Yare is very low volume. Severn has a bit more inside room. Weights for both as bare hulls should be under 30 lbs.
If you compare plans many of the hull panel dimensions are the same. The Yare is narrower because the shear clamps are simply pulled closer together. Because the Yare has a slightly deeper hull the deck beams have a shallow 24" radius. This makes for a tight fit if your feet are over size ten. The shallow Severn has deck beams that have an 18" radius and a more copious interior.
It wouldn't be hard to combine the longer Yare hull with the high arch Severn deck. Width could be 24" or 26" at your choosing.
The reason that I went thru' all this is that your idea of stretching the center is very risky. Chris Kulczicky claimed that he couldn't build this type of hull from his drawings and get what he wanted. Every tortured plywood design of his created a pile of discarded boats before he got one that was acceptable.
What I outlined above should work but I haven't done it. If I ever build another Yare It will have the Severn deck for the benefit of my big feet.
Caveat: The Yare is like smelly cheese. Some love it others hate it. It's tight and feels tippy. But it is a sea kayak. If you want low weight and sufficient hull strength either boat should be built with BS 1088 or 6566 plywood. There's only 3mm of wood between you and the water so use the good stuff.
Hope this helps.
Charlie

: I am contemplating building a Severn from the plans in the New Kayak Shop.

: I have recently finished a CLC Westriver 162 and now have a need for another
: boat so that I can share the fun a little.

: I noticed that the plans for the Severn show that the center section of the
: hull panels is cut parallel. I was wondering weather I could extend this
: section to lengthen the kayak (about a foot or so to help it keep up with
: the west river) without adversely affecting the handling of the boat.

: I was also looking for comments on the paddling qualities and durability of
: this design. (the weight specified has me both impressed and worried).