I am trying to reproduce 19th century engineering here. I need to knock off the corner of the flat cyan “solid” so that it fairs into the gray “solid.” Obvious they will never match up because the gray extends beyond the cyan at the sides.
However, the desired effect would be to knock off the corner to the cyan roughly matches the gray where these is overlap.
The curvature only needs to extend an inch or two.
Hmmm… If I’m understanding you correctly I’d copy the top edge of the back of the grey solid and split the curve and rejoin it so that you have the top curved part but not the long edges:
From there you can use a combination of BlendSrf and MatchSrf to get a continuous surface. But once you’ve created the basic geometery you can take sections and rebuild the curves into continuous elements and then Sweep/Loft a cleaner surface.