I just finished matching up a hull and deck with a similar problem, though not quite as severe. I cut some pieces to spread the narrower of the two pieces to force them open as much as I thought I dared. I then used a bunch of nylon strap clamps to pull the wide pieces in as much as I thought I dared. I ended up getting a pretty close fit, but I spent a lot of time doing it. This is an issue I've seen discussed on this forum quite often and I strongly advise everyone to take appropriate steps to hold the pieces in shape while epoxying. It's much easier to prevent this problem than to correct--as I now know.

: Hello,
: I'm a first time builder currently glassing a Guillemot. I have just glassed
: the inside of the hull and it has flexed open 1" while curing, if I
: had noticed the problem earlier I would have installed spacers before the
: resin was fully cured. My concern is that packing tape will not be enough
: when it comes time to join the halves. It takes quite a bit of force to
: squeeze it back to a 21" beam. Is 1" a problem? Will the process
: of joining the halves flex the whole thing back into shape, or should I
: try to fix this now? Would it help to install spacers and then add a fill
: coat of epoxy inside, or should I just try to get the deck to 21" and
: muscle the halves together when the time comes? Any advice would be much
: appreciated.
: -James