Absolutely glue them together first. You will notice that the deck surface does not line up perfectly. By gluing them together ahead of time, you can add some reinforcements under the deck using stock styrene. This will really firm up the deck and allows you to get everything even. And yes, you can still spread the hull apart enough to get it in. I did a complete wooden deck on a single sheet of styrene which was popped in and out several times to check for fit and lighting placement (yes, I lit the gun deck with LEDs). To slide the glued up deck in, start by putting the bow section in about 4 to 5 inches back - about where the opening in the gunwales starts. As you spread the deck apart aft, just continue to slide the deck forward until it meets the bow. In reality, getting the gun deck in is MUCH harder than the spar deck. All of my hints and complete build (thus far) are on the FineScale Modeler site under "Luvspinball."