why dont you scrape out a large amount
of the mortar parr a bit of wood to fit in
about 2 inch wide sticking out slightly farther
than required in 3 or 4 places on each side
fill hole with cement push wood back in
when set cut off wood to fit then hopfully
when you tighten up screws they wont
pull the wood out of the wall
this hopfully would give you the horizontal
securing required