I'd like to build my first AR early next year.  I already have a 20" barrel that's in great condition so I'd like to use it on the rifle.  Is it possible to order a complete upper from a supplier such as Bushmaster, DPMS or RRA MINUS the barrel?  Will they sell an upper or an upper kit in that configuration, or will I have to buy every component that goes on the upper separately....except for the barrel, of course?

Also, the barrel I have is a pre-ban.  I know I have to unscrew the flash hider and grind off the bayonet lug to stay within ATF guidelines.  What would you recommend to pin on the end of the barrel to cover the screws?  I'd like to get something that looks as close to the military flash hider as is legally possible.

Just about everyone sells both the stripped upper or complete upper minus the barrel - its on their respective website and in their catalogs, I was just looking at them today because I want to do the same thing. Fulton Armory has some nice M16 and M16A1 stripped uppers with kits, as well as A2 and A4 uppers.
I want to build a retro AR15 (SP1 style).