If the location of the door allows, make the vertical cuts 4" from the end of the block and you will be cutting in the "cell" of the block rather than hitting the cross web of the block. The thickness within the cell will be about 1" thick. However that may not hold true on some of the older block with a different configuration of "cells". Most of the block I worked with only had 2 cells.
Jerry
PS If you are using a diamond blade you can saturate the block with water before cutting----that reduces the dust like wet blade saws used in the masonry industry. I think you would need to use the shroud on a side grinder to prevent water or wet material being slung into the openings of the grinder.