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