Molesworth wrote:
.. I have a set of cast Danforth Davits. Have blasted and painted
them
and am now nearly ready to put them on the boat.

They are rated 175 lbs each.

They are going on the stern of a 39' 1980 center cockpit boat. The
fiberglass is very thick.

First, I'd like your collective input on how much/what sort of
bracing
in the aft cabin to support the davits with a hardshell dinghy
mounted.

First things first.

If you take a wave over the stern with the dinghy in place, 175#
rating/bracket will hopefully be the failure point and not tear your
stern out.

Your problem is stress concentraation where the fastners attach the
brackets to the hull.

Use some 15# urethane foam against the inside of the hull formed to
match the hull, then bond in place with SikaFlex 291.

Glass foam to hull after SikaFlex cures with some epoxy and double
bias glass, then use a 1/4" S/S plate(s) to distribute the stress at
the fastener penetrations..

SFWIW, I'd sell the davit brackets and keep the dinghy on deck.

Lew