Dash mount

Just use your head. I sawed mine off from below, so the screws were ruined. I just "guessed" I think I used 5/8" long screws ? Don't remember. You don't want the screws to impact the gasket much, or they might cause a leak.

Select screws that you can use a starter tool on effectively, deep hex head, or similar

Just reverse the clips. It's easier with two guys, but I managed to do mine alone, an empty shell of a dash.