I have this same rifle. I just mic'ed my magazine the other day for a project I'm planning, 3.589" is what I got. So yeah, around 3.580". Good thing you're planning to reload, I have had terrible luck with factory 300 Wby rounds. I've shot in the 1/4 moa area with this rifle with hand loads, but it has never shot any factory loads well. With one exception the 180 Hornady SP that Weatherby loads. It shoots it to about 1/2 moa. The bad thing about a Weatherby, or at least my Weatherby, is that it doesn't seem to want to shoot high BC bullets very well. I get my best groups with 180gr Nosler Partition and 80.5 grains of RL22, in my rifle that's good for 3120 fps. That charge is over the maximum, so work up to it. Oh, and good luck finding F215 primers... I haven't found any in years. That's one of the reasons I just had a 6.5x284 built.
And another thing, if you'll change that factory stock out for a straight comb stock, you'll get way less muzzle rise. Not that you should need a fast follow up shot with this rifle, but if you do, you'll be able to. Get steal rings and bases too. I've got Warn bases, and Burris rings.
I hope this helps.