Actual size of new const. window frame??
Based on the RO dimensions in your original post, it sounds like the problem is the RO height, not the RO width. Is that correct?
If the RO is the correct width, and you just need an extra inch or so of height, you might be able to lower the sill plate a bit. There's no harm in messing with it because it's not structural. It's only supporting the window.
My windows were framed with a double 2x4 for the sill plate, and I think that's pretty common. If you have something similar, I think you could simply remove the top 2x4 from the sill to give yourself an extra 1.5" of height. Interior and exterior finish work wouldn't be to bad, probably easier than if you went with a smaller window and had to get creative with trim. Plus, you wouldn't lose 4" of viewing space.