Whoa there! This is a big league structural issue when you start sawing structural concrete like a foundation.
The good news is it can be done, the bad news is that it takes the help of a structural engineer and a building permit is required.
You need the engineer to assess the structural loads, opening size and location in the wall, and the need for any lintel or beam reinforcement for the opening when it gets filled with a window or door.
You need the permit to comply with building codes.
Also think about the enormous size and weight of the hunk of concrete coming out of the wall and what to do with it.
Please email back what part of town in which this work will be done, and I can possibly refer you to someone who can help you with this.