I did just what you're wanting to do and the feed rails will need a little massaging. Actually had 2 done, one by Mike Bryant and the other by Chris Matthews, the latter was a 7 Rem Mag. Both feed flawlessly.
With a 300 RUM mag box and the feed rails modified you'll have no problems, you might have to have the upper rear portion of the ejection port beveled slightly also to ensure proper ejection.
Email Chris, he'll set you up.
Merry Christmas to you to.
Wayne aka WAMBO