Cranial...
I do believe that DPMS offers "lefty" (i.e, ejects to left side, not right) upper receivers
in A2 and flattop+handle versions, as well as lefty bolt assemblies. [Unless I'm brain-dead, I think you need a lefty bolt carrier.] I I believe you can get a DPMS upper now that's made from *forged*, not cast alloy (forged is best, esp for upper receiver which "does all the work").
DPMS offer whole rifles too, and they have a good reputation for quality and support. Unfortunately DPMS apparently does NOT offer mil-spec chrome-lined barrels. (Do not confuse chrome-lined with "chrome moly" or "Chromoly" - most AR15 barrel steels are in fact chrome moly, and this has nothing to do with lining).
Milspec chrome-lined barrels have military-sized chambers that offer far better chambering & extraction of ammo than "sporting" rifle chambers for a variety of ammo types. (Usually when you see a chambering, extraction, failure to feed problem on this board or the AR15 group mailing list it's often a rifle that has a non-milspec chamber, and a nonchromed barrel.) Chrome-lined barrels are far easier to clean, don't really have corrosion issues, and last much, much longer than unlined barrels: chrome is far harder than barrel steel itself and offers much greater "lubricity" (reduced friction).
So...
(1) get a lefty upper receiver & bolt/carrier assembly from DPMS. Send it to Bushmaster or Armalite and have 'em build up a rifle from this.
...OR...
(2) Get a chrome-lined Bushmaster, Colt or Armalite *postban* barrel in your choice of length (16" or 20") and send it to DPMS and have 'em build you up a DPMS gun w/chromelined barrel and lefty setup.
(3) You could also use Rock River Arms or J&T Distributing - great folks, great service. Just supply 'em with the lefty upper/bolt and a chromed barrel and let 'em take care of the rest.
FWIW, you might check with Gunsmoke Enterprises (http://www.ar15.tv) - I think they'll build you a Bushmaster at a fair discount, and will cooperate with your upper rcvr/bolt assy substitution.
Any of these ways you'll end up just fine.
Bill Wiese
San Mateo, CA