raise gothic arch in sandstone wall
 

raise gothic arch in sandstone wall
can anyone provide any suggestions -
i have to raise a gothic arch (approx width 1.8 meters) in a 600mm thick sand stone wall. the external surface of the wall is random square/rectangular cut sandstone blocks - the internal side is random shape stone work. i don't know the depth of the sandstone peices - but from other activities i know that some blocks will span the 600mm wall thickness
the peak of the arch is already 7 meters from floor level (discounting normal acro props)- and i wish to raise it by another 1.5 meter - any suggestions ??
thanks
phil