The reason for the gap is not just to avoid trimming exactly at the factory, the gap is there to prevent a "hard spot" where the hull touches the bulkhead. Fiberglass flexes and if the bulkhead fit tight it would ultimately result in a potential crack in the hull surface where the bulkhead presses against the inside of the hull. Some high-end builders will actually insert a foam strip between the edges of the bulkheads and the inside of the hull to cushion the area and if wedge shaped, tapering away from the bulkhead on both sides, will allow an easier application of the glass tape used to secure the bulkhead in place.
I would still use a wood or other material support to hold the deck up while the bulkhead is out, it may seem strong enough, and is, but the deck may still begin the sag down a bit without the bulkhead's support and you may actually want to slightly lift the deck up before installing the new bulkhead to make it easier to slip it into place. Several owners have left out the temporary supports while replacing bulkheads, only to have trouble with the new bulkheads not fitting because the deck has sagged and/or the hull spread slightly without the bulkhead in place.