I found that Mover's Rubber Bands are very handy for lining up the deck and hull, both for the initial adjustment phase and for the spot welding of the hull to the deck.  The giant rubber bands are cheap, reusable and the tension of each band can be infinately adjusted.  The rubber bands go under the hull.  The two end loops are connected across the deck by a zip tie.  For more tension, tighten the zip tie.  Amazon sells these in several lengths.