Asked & answered.
3/8"-16 thread, match length underhead. Hex head screws okeh, washers not strictly necessary (but not a bad idea, either.)
You'll find my accumulation of screw sizes on my site - start at www.kelleyswip.com/tech.html
, and follow the links from there. Questions like this are why I've been trying to amass that data and get it all in one place...
(NB: SAE5 or SAE8 will work, as will CRES. You don't need massive strength in that location. I've also had good luck putting studs in those holes, but it can be a bit of an exercise getting the nuts started...)