trish, the Egyptians built the pyramids using heat to break massive stones into uniform blocks.
If you get a tub of cold water ready, and get some lighter fluid and some twine that absorbs liquid (so not nylon etc) and soak the string, tie it over the corner at the angle you want it to be, light the string and wait till it burns low, dip the stone into the cold water and tap the corner section, it should break perfectly along the string line.
Do this on each corner and it will fit.
Although looking at your avatar choice, maybe you are not the right person for this job.
