First, this metal sided mobilehome is a 1973 Concord, but still in very good shape (only 1 ceiling stain from an old leak and no mold spores during testing)
The company that made the windows botched on the sizes (made them for rough opening instead of exact opening) so there is a gap around them (probably 1"x1" or 2"x1" will fur the gap) I'm adding siding with insulation afterwards so I'm not so worried about the looks at the moment (just not freezing my butt off this winter in Seattle!)
So what I am doing (and have done on the smaller windows I put in already) is removing the original window, then I cut the exterior sheet metal along the edge of the top of the opening (they wrapped the metal inside around the opening when they built these back in the day) and notch the corners of the fin on top to the edge of the frame. I'm thinking about adding that sill wrap tape for the next ones, then tucking the top window fin between the mobile's sheet metal exterior and the inner frame (which is probably 2"x3" or possibly 2" by almost 4" stud/sill) Then another piece of the sill tape on the outside to keep water/drafts from getting in. (it will all be hidden under the siding I plan to put on)
I got the windows made for 2"x6" walls with centered fins so they stick out far enough to be even with the siding that's going in.
That's as far as I've got so far.