Any hull that is built on a mold can be shortened/lengthend by changing the spacing of the cross stations.
Lengthening the spacing on a stipper mold has to be done with caution because the strips need a certain amount of support to make certain you get the intended shape.
No matter how you slice it a kayak is going to box in your dog. If you dog can't come to grips with the tight quarters I would leave it on shore. If the dog is a must build a stipper canoe about 18 feet long with a wide, flat bottom.

: I am hoping to build an open-cockpit boat that will accommodate my long 6'
: 3" frame, my wife and our dog. Is it possible to modify (lengthen) an
: existing strip design kayak by carefully spacing the patterns further
: apart?
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