I am replacing my current windows with new construction windows. Because I am in a historic district I need to keep the same size windows (opening size) and exterior molding. My plan is to remove the exterior molding up to the j-channel (leaving the j-channel), installing the new window, and then installing new exterior molding to match the current size that is there now. After I remove the old windows, the rough opening will be too large... i.e. I think if I purchase a new window to fit the rough opening, I would not be able to have the same size molding which is a requirement of the historic district.
Am I correct that I will need to build up the rough opening to provide a place for the new construction window to nail into and also allow enough room for the exterior molding? And what is the correct size windows I should order? Is 30" x 63 1/2" correct?