Pretty sure McGowen knows the specs for adding a barrel to the 700ML. IIRC the tenon is a different length than a normal Rem700 but they made them before. You want a 1-20 or 1-22 if going with a 45cal 8 groove.
If that's the case then it would just be a matter of unscrewing the old and screwing in the new because there is no headspace to deal with. Right?
Its not quite that simple. It will need to be head spaced if going the Hunter bare primer 209 setup. A Remage style barrel makes it easier for the DIY guys.