My first three kayaks had bigger gaps than yours but using a combination of the methods described here, I managed to get the deck and hull to align well enough that I only had to plane off 1 or 2 millimetres in a few places to get a smooth join.
The hull always curled in and the deck flattened out.

With my second and third kayaks, I tried preventing the changes in shape by keeping the hull on the forms. It wasn't enough to prevent the problem.

My fourth kayak was a stretched version of number three.

After each stage of glassing, I kept a number of pieces of timber wedged between the sides of the hull to force it out about 10mm wider than it needed to be. After glassing the inside, I kept a lot of rope tied around the outside of the deck to force it in 10mm narrower than it needed to be.

I hoped the technique would have the resin on the inside cure with a curve that would compensate for the tendency to distort.

The theory was good, but I overdid it. For the first time ever, I had a hull that was a bit wider than the deck. I got them to match quite easily, certainly more easily than my previous kayaks.

I have found that even after all glassing has been completed, if the hull and deck are forced out and in just a bit more than is needed and then they are left for several days it helps a bit. When the sticks and ropes are removed so the halves can be joined, the two halves don't spring back quite as much as the original shape.

I would have thought that with the size of your gaps, you should be able to get a very good join with a bit of effort.