this is a common issue with boats with "thin" hulls. the most important thing is to block the boat right in its cradle. When they are in the water they are fine. but on the hard attention needs to be given to the blocking. many catalinas, pearsons, hunters ect. of that ere are the same too. flex is normal but can be "scary" to someone who has never repaired boats and understands boat design. This is a coastal cruiser not a circumnavigator, not many boats are in this category. Many boats show small cracks in the hull to keel joint and this is from flex and stress, if the boat is sound then there should be no extreme amounts of cracks, just the occasional few.
