There is a potential for some fairly long shots ( 250-300 yards)..

For that reason I'd leave the 45-70 home...The .375 would work if you can shoot it well, but it's more than you need..Most .375s are fairly heavy, and there is usually a LOT of walking in diffiicult terrain in a Newfoundland hunt..A heavy rifle can be a burden..

The 300 WSM is probably close to ideal, but there's nothing wrong with the .308, either, especially it it is an old friend that you are very familiar with and can shoot well.... In any case I'd suggest shooting premium bullets like the Nosler partition or Barnes TSX ...