The receiver body is a little different width in the magazine well area though it can be opened up with a few strokes of a flat file. See if your 300 Rum brass will drop through.
I've converted a 7mm Rem mag 700 BDL to 300 Rum BDL. It wasn't difficult with hand tools but it wasn't drop in. I did buy a 300 Rum mag box and follower. I'm planning to replace the Rem mag box with a Wyatts extended magazine box. . A 3.6" magazine is very cramped for a 300 RUM if you want to shoot heavy or low drag bullets. Doing the conversion takes two short cuts with a milling machine. See WYATT'S OUTDOOR, INC.