Yes, that is a problem. The gap between planks in Lapstitch construction need to be COMPLETELY FILLED. If not, air pockets will trap water, inviting rot. Large gaps like the one pictured are difficult to fill. Unthickened epoxy will run out of the joint and thickened epoxy may leave pockets which are invisible to the eye but not to water. You need to close that gap as much as possible. I built an EP from plans and had the same issue. I was able to get the gap down to less then 1/8" and it still took three application of epoxy slightly thickened with colloidal silica before I was satisfied there were no internal voids. 

You will not be able to close that gap merely by twisting wires. Try placing  the bottom panel on two sawhorses placed at the very ends of the panel and then putting a heavy object on the top of the panel in the middle. This will induce a curve that will match up with panel 1#. You still may not be able to close the gap with wires because the forces of the curved panels are very strong. I end up screwing some blocks on each side of the gap and then used a big clamp on the blocks. I didn't care about the screw holes because that area of my EP is painted inside and out. Squeezing with the clamp got the gap to about 3/32", which was close enough. I then put a big "tack weld" of epoxy on the outer seam. Hope this helps.

Thanks for the info.  Good idea about the blocks and clamps!  I'm going to go that route as I alread decided to go with paint anyway. 

   Don't know if you are buiding from kit or plans. I built from plans and had the same trouble you describe. No matter what I did, I could not bring the gaps together at the bow and stern.

It turns out that I had used the wrong setting on the rabbeting bit. The rabbit was not deep enough. Re-rabbeting with the correct dimension bit solved the problem and had the boat wired correctly (and quickly) after the adjustment. 

May not be your issue, but worth a look at the dimensions on the rabbets.