Mate, I can't help you much with your seat solution but I have just put a full floor in my 4.2 WB Dory and thought you may be interested in some pic's.
As it was before doing any work.
DSCF4123.jpg
After having the frame welded in.
P1020456_1.jpg
Completed Floor.
P1020499_1.jpg
My thwarts have just a layer of High density PE foam, they have been very durable and comfortable.
Cheers,
Mike