Hi Derek.
Thank you for the question.
First off , all these pontoon based houseboats built here do in fact flex.
This is most evident when the boats are moored with the bows pushed against any uneven river bank.
In that respect you are quite right .
Most of the steel framed ones built here are lined with Gyprock ( US drywall ) to the walls and ceiling.
These boats have displayed sufficient flex to show cracks at the cornice or corner moldings.
What I did is epoxy glue and bolt the hull sections together using a string line to get them straight and true.
I was pleased with the result , they did in fact turn out to be quite straight and true.

Each hull is , in effect , a rigid box beam / girder .
So , longitudinally they are good to go for transport.
After the hulls were assembled they were chocked level with large timber blocks using a water level.
I did get them very close to level , that is to say the level did not show any discrepancies at all over the forty foot length.
That is good enough for me.
The 150 x 50 ( 2 x 6 ) timber beams ( there are 22 of them ) are stress graded ( visually inspected and selected by a professional timber grader ).
They were checked for wind shakes , checking , and allowable knot size and location , as well as moisture content and bowing and twist.


Stress graded deck beams.

The hulls turned out straight and true.

The deck ( floor ) aligns parallel with the water plane.

There is enough flex in the Whitco casement stays to allow movement inside the frame , and there is additional clearance from the opening frame to the casement proper.
There will be flex Derek , to be sure.I anticipate it will be ok for transport.
Windows slightly ajar during transport is a prudent precaution I think.