If you grind or saw masonry, remember to wear appropriate breathing protection. Silicosis is a very damaging disease.
Plain, cheap composite, masonry grinding wheels work well. If you can have a helper trickle water on the area, the dust will be far less. Remember to change your furnace filter when done. Dust will find its way there.
Plan B: The Easier Way...
We faced the same situation as you're facing- pre-made would not fit some of our openings.
I bought individual blocks at a big box store and mortared them in. By cheating a tiny bit on each mortar joint, I made up the 1/2" the opening was short.
I also was able to put in vents & those tilt-up windows where we wanted them. There is white "Glass Block" mortar available, but I used white thinset. I added a dryer vent port in a workshop window to connect an exhaust fan.
Actually, building the windows block-by-block was a whole lot easier than installing the pre-made ones. They are heavy and hard to make level and plumb at the same time. Also, after a few years, the mortared ones look cleaner than the pre-made ones that are siliconed together. (In my opinion, anyway. I like the white mortar joint versus the "used to be clear" silicone joints.)
Which every way you choose, be sure to enjoy your project and your new windows!
Paul