Andy, you can make lugs from sheet steel, .03 thick, and dovetail them in with no worries with that kind of wall. The lugs merely hold the stock tight to the barrel.
Please don't be pounding the undercuts with a great big chisel with a heavy hammer, you'll be sure to dent the bore.
If it were mine, I'd make an .025 deep cut. Then with a 1/8" wide chisel, raise up the dovetails with light blows, while moving the chisel from one side of the barrel cutout to the other, one blow at a time. Set your lug in place, then tap the raised metal down to retain the lug.