Hi Shuter,
The .375 H&H headspaces on the belt, therefore, you gain little to nothing toward success in hunting big game with this cartridge, by only neck sizing it.
Neck sizing is more for serious bench-rest target shooters and long rang prairie dog hunters, in their efforts to put 5 shots into one hole at 100 yds or even at 200 yds these days.
If your .375 will put 3 bullets into an inch and a half at 100 paces, from sand bags, and if you can hit a coffee cup from sticks with it, you will be fine.
Be that as it may, for a few dollars more you can buy the Redding 3 die set.
Better to have the extra die and not need it than to need it and not have it.
(You might want to shoot prairie dogs with your .375 some day).
I've used various rifles in .375 H&H in Alaska, Nevada, California and Africa (Ruger single shot, Pre-64 Model 70, Rem Model 700, Merkel SxS, assorted Mausers) and have never bothered with neck sizing for them.
Whichever method / die you decide to use, prior to any hunting trip point in on something that can absorb the bullet (backstop at a Rifle Range) and run all of your live cartridges through your rifle , to be very sure your ammunition will function smoothly.
Cheers,
Velo Dog.