Do I need a new window sill/stool?
NO, you may not need new window sills. Without pictures I can only use my imagination (and it ain't Disney's) to picture what you are into. Let's look at the scenario this way: On the outside of your sills, does the sill have a sort of drop of about 1/8"-3/16" that is 1 3/4" back from the outside edge? Most around here do. I call that the "nose" of the sill. IF you have this "nose", then imagine a straight line running side to side where this sort of drops of, and mark it accordingly. Now let's say you want to put in the wider Craftsman type windows, and they are wider than the existing sill. I use a Dremel Multimax and cut along this line form side to side until the nose comes off. Now fit the new casing/brickmold to the window and cut you a new nose piece the correct length to match up with the casing. Before securing the new nose piece, I use liquid nails basically as a sealer and then #8 galvinized finishing nails every 8". Set the nails and fill in. If I'm thinking correctly on this project this should work and look original. Good Luck, David