Re: 33-378 custom build
For a packing rifle, you have 4 easy options.
.338 Lapua AI
.338-300RUM (338 Edge)
.338 Lapua Magnum
The Lapua's have the best brass available and the AI is nearly the equal of the WBY (but much more efficient). The .338-300Rum (Edge) is the cheapest/easiest to build (you could use your 300Rum as a donor and just do a barrel swap. The Lapua's require a larger boltface and larger actions.
I'd look seriously at the 338 Edge, especially if you don't mind using your 300 Rum as a donor. A simple barrel swap and you'll have a rifle that will easily reach past 1000yd's for elk. It is a tad smaller than the 33-378 (within 100fps or so), but plenty to reach past 1000yds on elk.
If you are going with a complete custom and don't want the 338 Edge, I'd look seriously at a 338 Lapua improved before I went with the 33-378. The brass is fantastic and it is more efficient for nearly the same velocity.
Your choices below the 408 based rifles are the Wby, Lapua AI, and Edge.
Personally, I would not select a Lazerroni due to single source for brass.
Or you could go ahead and get a .408 based 338 and hire someone to carry it for you! I have a 338 Allen Magnum (.408 based) and it is an incredible hammer, it only weighs 18lbs with bipod and scope. It can be built in a 16lb configuration and is unmatched in it's long range capabilities out to 1500yds+.
If some is good and more is better, then too much is just right.
My mind is a raging torrent, flooded with rivulets of thought, cascading into a waterfall of creative alternatives