So maybe not too many Weatherby fans / users here? In cas it's helpful to anyone else, I've figured out that the 6-lug Mark V action is only made in one length, for "standard" cartridges (e.g., 30-06 length).

Found an ANIB Super Big Game Master in 338-06 with Redding dies (my personal favorites) for $1,125. That seems to be a pretty good deal, so I'm going to buy it under the assumption that I can sell it for at least that much if for some reason I don't like it.