Hi what I would do is to make up some deck beams to give the correct width, and temp fit them to stretch the hull out,then immerse it into a bath of hot water for about 20 minutes, then let it cool with the deck beams still in place,
When the time comes to fitting the support strips around inside the hull fit them in with the deck beams in place ,and put your permanent beams in at the same time.
This should give you a hull that is ok for use. Geo.