We have torn out a massive stone fireplace to replace it with
something smaller and more modern. My husband is constructing a box
around the remaining rough brick that was underneath the stone and he
is covering this box with cement board as the gas insert throws a lot
of heat. The hearth will be raised up and is black granite - one long
piece, 74 inches across, 15 inches deep. The granite is 3/4 inch
thick. It is very heavy! We have rebuilt the bottom of the box
several times now trying to get it perfectly level. As folks will be
sitting on this hearth from time to time we are concerned about the
granite cracking if it is not supported evenly & perfectly. Any
suggestions? Is cracking a concern? How to secure it to cement board
base? Thanks!

A Norris writes:

How to secure it to cement board base?

Thinset with a large-notch notched trowel. Notching the mortar spread will
allow you to squish down the stone here and there to get it level, but will
still support it solidly everywhere.