Hi Chris,

I had the same problem with my hull being slightly narrower than my deck. What I did was hotglue some small wood wedges on the underside of the deck. The wedges were spaced slightly less than 1/8" from the edge of the deck at 8"-12" intervals. When the deck was put on the wedges forced the hull to spread and conform to the shape of the hull. If after the deck was in place, the hull was a little proud I just pushed in on the hull to crush the tip of the wedge till the hull was exactly the right shape. I then did mini fillets between the wedges, after the fillets were cured I popped the wedges off and did a full fillet and taped the seam.
If you look at the pic below you can see the inverted deck with the wedges hotglued in place. The pic seems to be losing pixels as I transfer it from on free photo hosting website to the next as they shut down. Hope you can see enough detail to get the idea of what I am talking about.

Happy building!

Ken