x2 on the pressure wash/clean out first to see whats going on. I would eliminate that center stringer altogether (at least from livewell aft) and run some kind of keel stiffener and put some bulkheads in. That way you can have an actual bilge compartment in the rear, along with the new tank compartment. Hope the stringers dont need to be totally removed!
It looks like the previous owners stringer rebuild consisted of some wood glassed to the tops and thats about it...First thing i would do is make sure the hull is supported nicely, then cut the tops off the stringers and dig the foam out to have a better look at the tabbing onto the hull. At the very minimum i would tab the insides of the stringers to the hull, if the outside tabbing is stuck well. Go ahead and remove the chine "stringers" too. Prepare to use that yellow crowbar