I am going to build a AR-15 using post ban parts. What if any post-ban parts can I use to still be a legal rifle? I would like to use a preban A2 20" barrel on a post ban A2 Lefty upper from DPMS.

You can us just about any parts you want, so long as you keep the pre-ban features at bay.  Basically, bayonet lug, flash hider and threaded barrel.  And grenade launcher, if that's a concern.  You could use a pre-ban front sight base, as long as you cut off the bayonet lug.  Same for pre/post barrel and any threads on the muzzel end.  The internal parts are all interchangable, unless you count Colt's funny pin sizes.

Getting M16 parts mixed in to an AR15 is another story.