I am currently gathering parts for a 416 Rem Mag project. I am using a Win. Model 70 Classic action that was originally a 300 Win Mag. If I take out the spacer and make the mag box 375 length and use this bottom metal will I get 4 down? Excuse my ignorance this is my first Winchester.

Of course you would, with room to spare if you use a flat magazine spring like in the Pre-64's and the 300 RUM M70's.