That is the jamb extension that I referred to in the other thread... As I mentioned, it depends on 1) if it is attached to the existing window, and 2) what the jamb depth of the existing window is. If it is less than 3 1/4", you will need that either cut-back (ripped down) or replaced as the new window will sit in farther. Another option would be to shim the new frame out, but that present a host of other challenges.
An alternative method would be to down-size the window and get no nailing fin, and install it inside that wood, essentially using it as a buck frame like a replacement window wood... That method would cause you to lose some glass area and also necessitate adding stops to the interior (typically shoe or cove) as well as some professional metal-work on the exterior to get everything flashed properly.
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