i used #3, for the same thing. its working just fine.
And I might attempt to scribe this irregular surface with a compass on each stringer.In these cases, I leave an adequate amount on the bottom of the treads (say 1") then temp attach them at the top, but 1" high. make sure stringers are level to each other and plumb, some rigging support is required here. then I use a 1" thick board spacer on the ground to scribe the new surface on the stringer.
this will need to be done in the location of the new stairs