Straight from the factory postban, their standard XM15 E2S 20" HBAR will be about as close as you can get. The biggest difference, aside from the fire control, will be no flash hider, bayo lug, and it has a 20" HBAR instead of the government profile barrel. The Govt profile barrel is contoured down thin under the handguards. No one offers a govt profile barrel for postbans though many wish someone would. The HBAR will add a noticable amount of weight.
However, if you have a little more money burning a hole in your pocket, you could send your upper to Kurts Custom Firearms. They have their own section here in the industry forums. He can machine your HBAR down to the govt profile, and if you want, put a muzzle brake that looks like a flash hider (but does not reduce flash) on the end of your barrel. After that, your rifle will look and feel very much like the M16A2. Good luck!