Steve, you may not like this, due to the mess..... But I had to fit concrete forms up against a rough stone wall once. The first one I scribed the form,,,,,, miserable work........ The next one, I used my angle grinder, and cut a groove into the stones, they were soft limestone, and it was much easier that way........ Now, I was working where it was a messy area, and grinding stone is really messy........ but that could make fitting your mantle a lot easier. Chuck
Chuck Keysor (circa 1956)
PRT Alpha 60" x 144" (circa 2004)
Columbo 5HP spindle
Aspire 9.0, Rhino 5