Moving insert 10” out of firebox..

[JPT]

Hi, I’m a stone mason with a client who has an existing Mendota 44i propane insert in a full masonry fireplace. They want it pulled out approx 10-12” and I’ll be laying block and thin stone veneer as its new surround. My question is, should I be laying firebrick with refractory as the first course under, beside and above (with angle irons) as the initial surround (essentially extending the firebox) and then the block surrounding that, or is cement block and regular mortar laid throughout enough to withstand the heat? Any advice appreciated. Thanks!