call up some foundation companies and builders and ask who do they use to cut concrete. it is usually best to hire that out since they have the equipment and more importantly the know how ..... someone experienced can make it look simple to do
"You get what you pay for, and sometimes free costs more!"
We start with a standard diamond blade in a 14" gas cut-off saw, step the blade up to a "fat blade" to compensate for teh wide chain width, and then to a wet-cut chainsaw to finish the cut off. The chainsaw allows you to cut square corners w/o overcutting, keep nice straight cuts/jambs, and keep most of the mess/fumes on the exterior.