Thanks to everyone who weighed in with suggestions.
I seriously considered abandoning the castings and building the galleries from scratch. There are many excellent examples on other build logs that I could have used to guide me. Two things kept me pressing on: 1. many of the decorations I have been painting key into the metal castings. While I am confident that will be a future source of frustration (the castings are very brittle so bending them means breaking them) - if I attempted a scratch replacement I would lose all those references and have to painstakingly replicate them in wood. Ugh. And 2. I was able to get a pretty good fit on the upper (smaller) galleries so using them was not only possible but desirable. As I mentioned earlier the castings are attractively detailed.
I went back to work on the larger galleries using a modification of Maarten's suggestion. I was not ready to attach anything to the hull quite yet so I used modeling clay as a base and built the galleries on the hull of the ship rather than on my workbench. This introduced it's own challenges: CA creates a very brittle/fragile metal to metal bond and epoxy is really just a slimy mess; add in clay, an unstable base, frustration...well, you get the picture.
In the end I got pretty close to what I was shooting for. I'll still need to use some putty as filler and they need more filing/shaping to close down some of the gaps - but these are usable now (indeed, they fit just as well or better than the "official build" version so I think some of my struggles were grounded in the accuracy of the castings themselves).
For your critical review:
View attachment 216778
View attachment 216779
Now, to see what I can do to smooth out some of the metal to metal joints (though I think decorations will cover a lot of those).
As always, I am encouraged by the simple fact that you visit when the opportunity arises... I'll try to keep it interesting!