Go for option 2, I like the look of the 375 Wby as a round, & like many others would like to hear what it does with the 350 gr bullets. If you go with option 3 you not only loose the 3" of barrel but the weight of metal you take out in the reboring, all weight from the front of the rifle where you'll need it for the 470. If it comes down to it I think the 23" is a better length for a 375 as well, but you would loose a little velocity on a 375 Wby, not a heap, maybe only 40-50 fps. I did this when I had a ZKK602 rebored from 375H&H & chambered to 416 Rigby, it was still very shootable like this, & velocity was not an issue. In a thumper like a 470 Capstick, I think the lose of forward weight could be.
Steve