The best way to cut the stones flush with the rest of the fireplace is to use a angle grinder with a diamond blade. Depending on the thickness of the stone it may require you to cut from both the top and bottom.
Another option, depending on the rest of the fireplace, would be to score the top, bottom and sides of the block with the grinder, no more the a 1/4" on all sides, then use a wide chisel and light tap it around the stone in the new groove until the stone cracks off. This will leave you with a rough natural look.
Hope this helps!
Andrew Pellak Jr.
A. M. Pellak Construction, LLC