What size and quantity of screws do I need for the:
Im using the U Drive screws for the rest of the build will I need any other screws?
What's up with the center support, I can't find info about it.
What is it, where do I get it, how do I put it in, does it exist?
The center support is riveted between the bottom rails and keeps the receiver from flexing. I believe Cope's (among others) sell the center support and rivet. Don't let the rivet scare you, they are easy to install. Just make sure you install it the right way. The selector must go over the top of the rivet head.