Re: Keel bolts
Mild vs stainless keel bolts is a long running question in yacht design, with good designers falling on both sides of the question. Both camps have good arguments, and both have their weaknesses.
The one problem with making a switch from mild to stainless without redesign is that the strength of stainless is much lower than mild, and can lead to breakage.
Going the other way can have galvanic issues depending on the keel material, and installation method.
Presuming the builder has a recommendation I would follow it, or at least clear it past them before making the switch.