Randy,
A couple of thoughts here.

If you use hot-melt glue to secure small blocks of wood on inside or outside, then work on the inside with glass strips between the blocks. knock off the blocks after the resin hardens and sand the edges of those short strips. It will look like a dotted line. apply another layer of glass fqbric over this segmented joint to give a complete seam. It is on the inside, so no one is going to notice the slight thickness variations of the glass,but it that bothers you, then patch in the spaces where the blocks were, and fair the patches smooth. You might not need a second layer of glass strip.

Are you sure it is the width of the deck which is your problem? I can understand your desire to not plane through your stained finish, but remember that the deck and hull are three-dimensional shapes. Basically they are triangles. One is pointed up and one is pointed down. The bases of these two triangles join together. If you reduce the height of either triangle you are also reducing the width of the base. If you plane on the gluing edge where deck and hull meet you clean up that joint for good gluing, but slightly change the width. This edge of the wood is not stained, so maybe you can tweek things by hand planing a sixteenth of an inch off the bottom edge both sides of the deck. You may not need to do the whole length-- fix the overhang area and blend that back a foot or two on either side.

Are you sure the length is right? What happens to the problems with the width if you use a long 2x4 and some wedges, bolts or blocks to stretch out the deck (or hull) lengthwise? You can place the 2x4 inside the boat and push things apart, or you can put the hull or deck in traction: build a rectangular frame around it and use cables to pull it longer. If you have already installed holes or a sealed tube for end loops, use that. At this point the boat is going to be quite flexible, so you some leeway to manipulate the shape.

Good luck with this

PGJ