Help to widen ships hull

How much is the interference?  Another way to handle this is narrowing the deck. I have done that.

As far as widening the hull, I have done this by about fifty to sixty mil by cutting  square stock of styrene to the correct width and inserting it as a "bulkhead".  The hull gave enough to let me insert the piece of styrene, put in at the widest part of the hull.  The piece must be completely below deck level, of course.  Then I added the deck and everything held fine.

I frequently have to alter the planform of decks slightly to eliminate gaps in the deck-to-bulwark joint, and for small alternations that works well.  If the problem is 1/8 or 3/16, then you have a serious problem.

Usually these kinds of things have unforseen "ripple" effects. Therefore avoiding any significant modifications saves you more trouble down the line.

Don't try heal to solve it- it will just lead to warpage.

Don's idea is a good one, it's commonly used in aircraft modeling to get the fuselage to fill the gap between the wings.

I have never built that kit, but I also have not heard of that problem in a brief search through reviews on the www. Double check and be sure you have the assembly right. Maybe the deck is supposed to sit on top of the hull. That ship DOES have a funny looking connection there, sort of a step.

Just my two bits.