This would be for a hunting rifle, long range rock busting rifle.
I would like to use the BDL metal with the biggest mag box available but am also open to chambering for single shot if that is a noticeable performance gain.
I am thinking a 28 inch barrel in the original contour.
I am leaning towards the 338 rum.
Your description is pretty much what I came up with while asking questions. Simple things first:
Selecting the .338 RUM means you need a new box and follower anyway. The rails will need to be adjusted as will the feed ramp.
The factory BDL will work if you keep the OAL to about 3.600" for the magazine. But you have the alternative to use one of those Wyatt boxes which will require cobbling your receiver to accommodate the length of the box and feeding the longer cartridges. You will also need a new follower.
I am personally comfortable with a single shot, unfortunately, others may not be as comfortable.
I think the 28" barrel is about as long as most of us would be comfortable hunting with. If this rifle was relegated to a bench or prone shooting without much walking and carrying, then the sky's the limit on length.
Given the capacity of the cases suggested, I believe that the best weight is still the 250 gr. VLDs, whether for targets or for hunting. There is not a significant increase in powder and energy to provide the horsepower to drive the heavier 285/300 grain class bullets. But, no one is saying that it can't be done, because it's being done every day, just without the best performance available.
Regards.