deleting partial patterns (stones) at the selection border (road)

I'm trying to create a filter for a medieval cobble stone road. The idea is that I draw the structure of the road map in Photoshop and use that as the selection for Filter Forge to create the stone pattern. Filter #14157 Stone House by Carl could be a good starting point.

However, I don't want partial stones, just cut off by the boundary of the selection. I want natural, complete stones only, creating a rough, natural road boundary. I imagine I could detect the stones that cross/touch the selection boundary and delete them/make them transparent. Any idea how I could do that? Or another approach that would lead to the same result?

Thanks in advance,

Joost

I see what you mean. You create the basis for the stones upfront, instead of deleting parts of it later. That makes more sense. Thanks!