I remember having hours of fun with the floor boards of an Avon sportsboat as a kid
Try an oar under each side with the boat just inflated (i.e. just enough pressure to give the tubes a little bit of shape.) it seems to help align the boards and allows the stringers to go in more easily, its still a pig to do
I think tube pressure needs to be a bit less than suggested above, 2-3psi ? These older Avons are not designed to be as rigid as modern boats and they do flex a bit (disconcerting if you are not used to it), but they perform well and seem to have good longevity.