I try to use the black mounts from Great Planes. After countersinking a small hole, I use the cheap 6/32" socket head screws that the hobby shop carries. They will never vibrate out, and it's a light and easy assembly. I also use a small washer under the screws, but I never really pay attention to grade, etc. I've had a couple of crashes and the mount always came free first, usually taking the firewall with it. The screws never broke and one crash was right on the spinner doing 70 or 80 mph. They have bent some, but have never broke.

Honestly, I wouldn't worry about this part of the mount too much.

I don't like the cheap blind nuts. I do check the threading on those and the bolts I use to get a good fit. I also swab some epoxy into the threads of the blind nut when I'm using thinned epoxy to fuel proof the area. I've not had one vibrate loose since doing it this way. I have had problems here in the past, but not with engine mounting screws.