I use #4-6 3/4"-1" socket head sheet metal screws on anything from a 46 size up to and including a 120 4 stroke. I have been using this since 1980 with no problem. Occasionally vibration will loosen them but a little blue loc-tight does the trick. When I got into RC back in 1980 some of the old timers back then told me about Sheet metal screws which is so much more easy to drill out the mount and R&R an engine. As to mounting the motor support to the fire wall, well I use what ever plans call for using blind nuts and blue lock tight. In determining what size screw to use, #4 or #6 depends on size of engine mounting hole, then drill out one size smaller drill bit then the drill size. Screw lenth depends on thickness of the mount whether I use a 3/4" or 1". That gives sufficient meat in the support for the screw to tighten into. In over 30 years never had a problem but to each his own.