Odd that it would be a box construction, but of course when you tighten the rigging
the mast is compressed or pushed down and the hull
is pulled up at the chain plates making it appear that the deck step is depressed / compressed by the extra mast loading.
The mast loads are transferred through the deck step to the compression post.
What you first have to do is asses the situation. I would slacken the rigging
so that there is little tension on the mast (don't go sailing like this!). Do this at anchor
or dockside. No take precise measurements around the compression post. If there is a bulkhead near w/ a door, look for the margin around the door when closed. Is the frame square and true? Since the joinery was done with no mast or hull tension you need to first try to get to the no load condtions and take measurements and observations. Use a digital camera
to documents as many "areas" as you can.
Now properly tension the rig. There is a tutorial on the Selden site with a link found on this site. You need a two meter or so length of wood or metal and a micrometer and some tape. IIRC each millimeter that you stretch the rigging adds 5% of breaking strength loading. You want to tension the rig to 15% this means that the upper shrouds have"stretched" 3mm. This does not mean that the turnbuckles have been screwed 3mm. The 3mm change is measured using the change in OAL length with refernce to the fixed (taped on 2 meter stick).
Now go below and take your measurements, obsevations and photos and make an assessment of how much distortion has occured.
If the compression post is not strong enough it will begin to crush under the load. This means that the force is effectivelly shortening the OAL length of the compression post and allowing the hull to list at the sides making the mast step seem more and more depressed. If you were to over tighten (if that was possible) the step would get lower and lowe, the chainplates higher and the beam reduced.
You don't want any crushing of the post under the 15% or more sailiing loads. If this is the case then you need to re inforce or replace the post. If you can remove it you'll be faced with getting one in that fits tight and has no gap at the underside of the step or at the keel.
One approach is to you a jack to push the deck up enough to slide the post in. This is not likely with a large cross section post unless there is sufficent clearance to allow you to do this. You may have to build up the area where the post is to seat on the keel. This must be rigid and non compressible. With the new post AND the build up made to the precise OAL height you can slide the new post on place (with the deck jacked up a few MM) and then release the jack and the deck will now return to the design height. You should be able to now re tension to 15% without distortion. Use the right material and cross section for the post compressive strength. Make sure it does not sit in bilge water
can occur. Make sure you have perfect mating at the base and top of the post and lateral restraint as well.
If the post abuts a bulkhead secure the post to the bulkhead the entire length which will distribute loading to the huil as well as the keel and there will be less distortion at tensioning and sailing loads.
This is best done in the water as the hull shape changes from when on the hard
On the hard the keel pushes UP distorting the hull making the compression post distance smaller. In the water it pulls down and this may lengthen the post distance!