I built mine in the early nineties, so its been awhile. If I remember correctly there are metal pieces that have to be attached underneath the sections so as to line them up. If my memory is wrong on this then I would find some way to do it with heavier sheet styrene cut into appropriate size strips. Then I would weld the 3 deck pieces together with Tenax 7R making sure they are as flush with each other as can be.
Then the seams: use masking tape on both sides of the seam (maybe a couple of pieces on top of one another so as to avoid sanding off detail) leaving maybe a 1/16 of styrene exposed on either side of the seam. then sand with a sanding pad made of a flexible material (maybe a damp sponge)
Take your time and get it right. Of course if there is a gap here or there just fill with your favorite putty. If a tie down gets sanded off. just use a piece of styrene rod to replace it and sand down to size. Did I mention to take your time? Then prime first then final coat.