If it can't be rotated (top aft, can't see if constrained on aft sides) then best to cut it horizontally to allow it to come out.. It may be that the piece below the lowermost horizontal stain could be cut horizontally on the stain line and when re-fit, a trim piece could cover or ya could veneer it after smoothly joining the parts.. Looks like that bulkhead is mostly loaded in compression, so a new tight fit would be fine.
Not familiar with that boat, but usually the hull/deck joint glued and screwed and pretty rough to get undone.. To get the bulkhead out whole, you'd probably need about 6 feet of loose hull/deck seam on both sides of bulkhead then carefully jack deck to release the bulkhead. Kinda dicey