Setting the correct width deck beam at the correct fore/aft location (and flexing the sides in as needed) will be setting the boat up in its per-plan shape and width.  No suprise that the highly flexible sides at this point in the build aren't naturally just sitting there at the per plan width.  Does the plan call for a screw from the outside of the hull panels into the deck beam?  If not, you can use a bronze or stainless screw to help hold everything together while glueing, even countersinking if you want to cover the head with putty to make it invisible when complete.  No need for the screw if you don't want to use one or the plans don't call for it - just use a ratchet strap or something similar wrapped aroung the whole hull to draw things together.

I'm not going to take the time to dig out my Chess 17 plans, but if I remember correctly the deck beam on it called for a screw - I decided I didn't want to use one and that all would be strong enought without it once the deck was glued on.  I actually regretted the decision because as soon as the glue was applied everything got so slippery that I had some difficutly keeping everything aligned and the deck beam standing square across the boat and vertical without a lot of fidgeting and tape and etc. while waiting for the epoxy to set.

deck beams on any of the chesapeake series should be put in their plan location as they are set-up to support the front of the cockpit/coaming.  so if they are not where they are supposed to be, it can get pretty wacky as it starts to impact the deck, cockpit, and coaming positioning.

it's also natural, as bubblehead mentions above, that the sides tend to bow out a bit and need to be pulled together to assume the correct width....especially if the hull is being supported from the bottom vs supported in a sling.   i have not built a chesapeake in a while, but screws were used at the time to hold these in place in addition to the epoxy.  you do, however, need to countersink them to not make a nub on your hull.

h

This topic came up for discussion recently among some of us assembling Waterlust canoes. Seems that the #4 frame, having a considerable portion 'missing' in the middle (otherwise it'd end up dividing the cockpit into fore & aft sections) leaves the assembled hull portion to relax to a wider dimension than the deck piece at that point.

I raised the prospect of using an improvised 'spanish windlass', secured with c-clamps at each end fixed to those frame segments, to bring the hull width into what'd be a better fit for the deck when that's ready for bonding. Another buiider used a ratchet tie-down similarly, while a third simply let it be then worked a generous radius into the deck/hull junction so the small 'shelf' he discovered simply ceased to exist.

Bringing Dillon into the discussion gave us confidence that all of these solutions are practical choices to bring about a satisfactory assembly.