First off your decision, but I do not see the advantage in any of your choices.
All require custom reamers, custom dies, and two require custom larger actions probably. 416 Brass is not cheap or easy to find in many cases.
All of them are "odd" choices to say the least IMO. ie, not common and no perceived advantage vs expense and difficulty.
Why not the 300 RUM instead of 30 STW/8mm Mag?
Of the 3 you posted, if you can find the weatherby action, the 30-378 would be easiest I guess my most favored.
For the flattest 30 cal, I would look at 30-338 Lapua or 30-338 Norma (standard long action for the Norma). Here is a link on the 338 Norma
For the flat 30, it has to be set up for the 240 SMK with 1-10 twist and 30"-34" barrels.