Apparently not. The issue we're having is that while the mantel is square, the wall and especially the fireplace are not. So we have gaps. Some of them are small and some are larger than we'd like to admit. We had planned to fill these with some caulk like This Old House recommends but apparently it isn't very heat resistant so I'm not quite sure what kind to get. Any input would be appreciated.
The other issue we ran into was that the bit set I had for masonry holes includes a 1/8" bit and not a 5/32" bit. This actually made a big difference and after getting a nice long 5/32" bit at Home Depot for $4 I was able to create pilot holes very easily for the #10 wood-masonry screws I had purchased.
Fortunately, the masonry screws were very strong and the mantel sides are not going anywhere. Unfortunately, it took a few tries and there are a few extra holes that we’ll have to fill and paint over.
Now for the shelves. I had previously made one shelf based on math before I realized that doing even the best math and measuring was not as accurate as just marking a line on the board, but I still have 7 to make. Hopefully the imperfections in the boards and the walls will be able to be fixed and everything will look good in the end.