I might have just done something very foolish. My new acquisition is a "Flagged Mauser" in 400 Whelen. Not sure what flagged means?

http://www.gunbroker.com/Auction/ViewItem.aspx?Item=402758680

I've been reading Elmer Keith's Gun Notes and he mentioned the 400 Whelen several times. I just love the old stories of ridding around for 2 or 3 months in the Yukon on a shooting holiday and it seems a Whelen always factored in somewhere.

I found 400 Whelen basic brass on MidwayUSA and I assume I need to do a chamber cast and figure out what dies to order or get made. Other then this I realize I'm kind of in over my head. Do I need 300gr .411 bullets for the 405 Winchester or can I use 450-400 400gr bullets. Where do I get reliable load data?

Any guidance would be welcomed, I'm over my skies on this one.