Have you called Anderson to see if you can have the windows made custom? Have you looked at Pella windows? If you have a good contractor he should be able to help with a solution. I would contact Anderson and or Pella directly.

Here is a link that might be useful: pella windows

The standard Andersen casement, with no extension jambs, has a 2-7/8" frame from the nailing flange to the interior. Actual tip to tip on the frame measures 4-3/16", but 1-5/16" is on the outside.
This is to allow for a drywall return, instead of casing.
Hope that helps.

Thanks for that info, mike35. Do you know what the dimensions are for Andersen double hungs & sliders? How about coreesponding dimensions for Marvin fiberglass & Marvin wood?

Andersen's double hungs and sliders are made with a 4-9/16" frames as standard with 2 exceptions. The 200 Series Tilt Wash can be ordered with no extensions, same as the casement, for drywall returns. But only standard sizes, and very few options. The Woodwright Insert replacement window comes with a 3-1/4" frame, made to sit inside an existing frame. It's even offered with a zero degree sill when it's replacing a casement and the existing frame has no slope.
All of Marvin's products come with a standard 4-9/16" frame with exception of the Ultimate Insert window, and Marvin's Infinity line of replacement windows. Those have a 3-1/4" frame.

I had steel casement windows, and I replaced them all with vinyl replacement windows. I did not want vinyl windows for the same reasons you listed, but with a lot of internet investigation, and several trips to local family owned home improvement stores, I found that not all vinyl windows are made alike or look alike. The old windows were removed and the new ones put in very easily. I kept the sills on all the windows except for my very large picture window, which i replaced with a bow window. On that one the sill had to go.

Good luck with your decision.

Abbey

My house has the steel casement windows installed in concrete. Should I cut out the glass then trim the remaining frame leaving only the approx. 1" frame around the opening to install the new windows up against the old steel
frame?