granite hearth installation

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!
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A Norris writes:

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.
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