"I watch a lot of fix-it and decorating shows so this suggestion is not from personal experience."
You need to find a way to build up the uneven surface not take away from it. I'm sure your building supply store could give you some reasonable options for the necessary products.
I think I would build a frame to match the frame you want to attach to the fireplace. Then I would take that frame and butt it up against the fireplace where you want to attach it. Then I would fill it in with the product from the building supply store (cement product and the anchor bolts) After it has cured, I would remove the frame and there is the smooth surface for the new doors to attach too.