If the engine is held with adequate clamping force, the threads will not be able to worm out the holes in the mounting lugs. There are limits to various methods for what works, real experience is the best teacher. Especially in competition where there are others with more experience who can show you what is needed to work and what is unnecessary over-kill.
The worst kind of "teacher" is the type who thinks he knows it all, especially when it comes to things that he has never tried.
Someone said earlier to use #4 hardware for .40 sized engines. I don't, I use #6 for .32 engines up to .50s. I've had the heads of high grade #4 machine screws pop off too many times, so learning from that experience, #6 hardware has rarely failed me on this size range of engine.
It helps to have a numbered drill index to custom match the pilot hole size to a sheetmetal screw. Some wax or soap on the screw threads helps them cut into the plastic or wood.
The Du-Bro or Great Planes socket head sheetmetal screws [SHSMS] work great if applied correctly.