I ignored the screws in the picture. I use the #6-32 hardened hex socket head screws and washers with Tee nuts in the back of the firewall.

I've been successful with drilling and tapping the beams for the 6-32's threaded directly into the beams. You do not even need lock washers or nuts. I put flat washers under the screw heads though.

There's a person that BLW and I know that uses sheet metal screws to hold the engine and I've never seem him have a problem with the screws but I don't like the idea. I've had too many regular grade screws fail in the past. They look like they would back right out with the coarse threads but his don't. I saw where he used the sheet metal screws into wood and they wouldn't last much more than a minute.