Yes im using the standoffs and screws that came with the 300. Nope not over-tightening at all I know im not suppose to. I cant over tighten what wont tighten in the first place. lol
Not using screws with teeth under.
Ok so the standoff's screw into the motherboard tray fine, but the screws that are meant to hold the motherboard into the standoff's are not tightening....
But these screws your talking about have no teeth? you mean they have no threads? are they even screws? find a pic online of the screws your talking about, because it sounds like your using nails!
Anyhow, try different screws. Easiest way would be to have a loose standoff in your hand, and go through all the screws provided with your case/motherboard, to find whatever best fits.