A 2" screw would be fine. Something like this would work. http://www.dhcsupplies.com/store/p/2...s-2-to-3-.html I like square drive since the bit wedges tighter if you push, and doesn't cam out like a typical phillips head. Do use some good construction adhesive on the joists. Then, the screws are there as clamps, almost irrelevant after the glue sets, but do add some strength.