I wanted to add that when I mate a deck and hull I start by taping the bow and stern ends together. Then I wiggle the hull/deck sides to mate with a little pushing and pulling and fiberglass packing tape.

When you say 18 inches off, I can only imagine you have the deck placed in a cocked position from the center out which is being compounded with some hull distortion.

Think of it this way. Get some forms in the boat and get it symmetrical. Then tape the deck ends on and if you have to add material on a side or shave it a bit you can do that. But you have to get the hull in shape first.

There.s nothing worse than a boat that pulls to one side. Trust me, I've done it and repaired it.