And what do you do if they don't fit?

I make them fit with bute force.

There is enough flex in the halves that I pushed the sections coplaner.
There is clamping pressure that the friction between hull and deck holds the
the alignment adjustments made by hand.

Also, the glassed hull and deck remained on the forms as much as possible to
help retain the proper shape. Epoxy shrinks as it cures and would otherwise
tend to warp the halves.

-Steve