mllm,
Why would you want to take a well functioning and accurate 223 AR upper and change out the barrel. Given the low cost of an upper receiver and bolt carrier group, etc... just buy new parts.
JDJ makes a very nice 300 as do several other custom gunsmiths, but you are paying for a custom gun. A custom match grade AR barrel will run you about $400 to $500 plus another $300 to $500 in parts (upper receiver, bolt, bolt carrier group gas block etc...).
There are other sources, however, that make very good semi-custom 300's for about $400 to $500 complete.
Try Toni at Del-Ton Inc. He has been mentioned on this site. I have one of his uppers and I am very happy with it.
http://www.del-ton.com/AR_15_Convers...ball_s/105.htm
You can also try Model-1-Sales. Check under "barrels"
http://www.model1sales.com
320pf