[villagetinker]

Bjeanj, I did exactly what you want to do on 7 or 8 windows up North. The house had the old single pane aluminum frame windows. I bought standard size replacement windows, which were the exact same width, but about 6 inches longer. I use a reciprocating saw and was able to cut out the exiting windows (aluminum siding), and then cut the additional opening for the new windows. Since I did not cut the original header, all I had to do was install a new lower 2 by for the window support. I added insulation before installing the 2 by and then the window. "J" channel was used to finish the outside siding. The inside was finished with 1/2 inch trim boards and suitable molding. The estimate I had was over $500 per window installed, my cost was less than $150 per window. The cost savings was I used standard size windows and adjusted the opening, custom windows were much more expensive.
Send me a PM with your phone number if you would like to discuss further.