Interesting idea.
My first thought is toward weight distribution and stability. Add a motor... And rainwater.
My second thought is toward dynamic loading; it's one thing to mount the davits on the hull where you could hope to rest assured that things are heavily-built. It's quite another to follow the loads down through a structure that is relatively lightly-built.
My third thought is to go back to the manufacturer, even though a 390 is an older model, and there were several versions of that boat over the years, and the ownership of the mfr has changed (I recall).
And last, I Googled 'Mainship 390' images and note that there appears to be a variety of arrangements of metal support posts under the flybridge overhang, from none to two of various appearances of heftyness and location. (Your pictured boat has three.) Adding posts of the correct heft and connection to hull below and through the flybridge to the davit is probably the key bit of engineering, presuming the stability is not an issue.