Mozzyuk

Hi there. Im in the process of having my fire converted from gas to coal/logs. the chimney has been swept by a chimneysweep and tested and the gas fire has been taken out, So im left with a hole apprx 21 inch deep to the back wall. So the question is this, Do i fit my 16 inch clay fireback touching the back chimney wall. If i do this i will have 19 inch to the front opening to put grate etc. Or do i slide the fireback forward towards the living room and fill the gap behind?. Also what about the side walls inside the hole? Could i skim the brickwork with cement? would this be ok heatwise, Or wo