You could just rebore the 06 barrel and be done with it. Save $ and if it has sights you are all set. A 24" barrel works fine and the scope is among the best in it's price range, although I might lean toward a 1-4 for big game as it's not a long range cartridge. BTW it also LOVES cast bullets!
Check with High Plains Reboring in ND. He did a 300 H&H 700 classic to 400 H&H for me and a 25-06 Browning high wall to 40-82 and they will both dump 3 in 1 MOA.(it's a .411 40-82 which in this action exceeds the 400 Whelen)
Here's pics of my 2 9.3x62s...... about 6 decades apart and the 400 H&H.
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