Can 375H&H brass be resized into other calibers like 338wm or 458,7mmstw,etc.?
Thanks

Absolutely, into any case that was derived from the belted H&H case (.300 H&H and .375 H&H). Short magnums (by short magnums here I am referring to belted short mags derived from the H&H case not the newer Win and Rem short mags) like .264 WM, 7mm Rem, .300 WM, .338 WM, the short Weatherby magnums, etc need to be cut down and then run through an intermediate sizing die first. Full length magnums such as .300 Wby, 8MM Rem Mag, etc are much simpler to form. It's a lot easier if making short magnums to pick a case that is close to what your are wanting to make rather than to use full-length H&H brass, for example I have made .270 Weatherby brass by running 7mm Rem Mag through a sizing die.

Thanks
whats the best way to cut brass?

I've locked them into my Forster case trimmer and then used a Dremel with a cutoff wheel. I've also used a hacksaw and a tubing cutter will work, too. If you have a case trimmer you can also use it and just trim them back to the length you want but you'll have to change mandrels partway through the process to keep it straight. Also, if forming new cases from cut down brass you will need to anneal the necks once you are finished and have them formed and trimmed to length.

Blackbear,
If you have a significant pile of .375 H&H brass no cutting is needed. Just offer to swap it for whatever brass you need. Someone will be happy to get it and will make you a deal.