Or is this too much a stretch? I’ve got 25 378Wby brass but had sold the rifle. Need 460 Wby brass and was hoping I could neck size them

I'd anneal the necks and fireform the cases with some cream of wheat loads. MuleDeer can steer you a bit closer in the Ask the Gunwriters section.

Yes but best if you first neck to 416 first and make sure lube inside neck. The Wby shoulder will collapse very easily like the 40 degree AI shoulders. The same applies to necking down, that is, one calibre change at a time. I have necked 416 Wby to 338/378 bit had to go to 378 first.

I agree with both responses above. I made 375 WAI brass out of 30-06 brass by opening up to 416 then back down by sizing a little at a time - 8mm, 32, 338, 35, 375, 416 then back to 373 (I needed to go up and back down to get the shoulder right.)

But since then I've used 35 Whelen brass to 375 and then COW

I didn't need to anneal, but it is a very good idea, I think.

Follow Mike's suggestion.
Because the .338/378, .378, .416 and .460 all use the same basic case with only neck differences and because the cartridge head spaces on the belt, there is no need or any benefit in fire forming. It would be a waste of time, powder, bullet and primer.