Back in my 40k days i always thought about using epic scale stuff to create a friese like this. Very nice.
Suggestions:
Make it a littke shallower.
Use clay to make a rough background and flatten the side of the minis that you wouldn't see in the relief then gently press them on.
Use some plasti strip to create the frame around the piece.
undercoat using grey paint with some thinned coving adhesive or really fine wall filler it will give a nice stone 'key' to paint on.
good luck