Re: cement block cutting
Diamond blades in both a circular saw and angle grinder. Mark you lines in ink or heavy pencil. Tent and ventilate the area to the exterior if indoors. Cut from both sides for your perfect hole, then use a chisel and hammer or rotohammer to remove the center. You may want to drill your corners for reference points on the opposite side of the wall.
A couple things you may run into are rebar and filled block cavities. For these you'll definitely need the rotohammer to remove the cement and then a sawzall to cut the rebar.
I can't advise whether you'd need to install a header (lintel ?? ) for the opening or not, nor how it will structurally affect the wall, particularly if this is a structural wall that you're modifying.
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