How can I modify Sash window frames??

Hi,
Need some advice on if it is possible to modify the 2 existing sash window timber frames in a double hung window setup.

I have picked up a 2nd hand double hung timber window with the old steel weights and cords setup in good condition.
The window is the correct width but approx. 400mm too high, so I am thinking to cut down and modify the 2 sash windows if possible. I have replaced sash cords and window glass before so am OK with this part of the work.......the top part of the window frame can be easily pulled off and reattached after the 2 vertical stiles have been cut down to the correct height.

The question is can anyone advice on how to cut down the height of the 2 sash timber frames without damaging mortice+tennon joins??.........I need to reduce height of both sashes by approx. 200mm each

I have demolished old timber sash frames before and these come apart easily by pulling the corner tenon out from the mortice after a few wacks with a hammer. I figured after cutting down the 2 vertical sides of the sash timber frame the only thing I need to do is recut new mortice holes an refix with glue.
Question ........will this work? .......what are basic tools required to cut mortice?

(the cost of new window which I need again with cords and weights is approx. $900-1000 so I think it may be worth modifying old frame)

cheers
:roll:


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