DD:
Pity the .404 Express FNB is too long to be made from the .338 brass, its certainly the easy way to go for the Taylor chambering. Also, the .300 has to be expanded quite a bit more than the .338 to make .416.

You could always try necking down the .458, and then external-spinning the neck on a mandrel in the lathe, as suggested by Nonte. It only has to be extended 0.120-inch, probably only one light pass. A lot of mucking about admittedly, but some folks love the occupational therapy!

Scoping the Taylor should be no trouble, my mate has one scoped in a plastic stock, and he has no problems. Not the place for a cheap scope or mounts, though. Leupold on up should be OK.