Assuming your Rem 798 in 375 H&H is the same as other Zastavas in 375 H&H you likely could fit the barrelled action if you modified the B&C Medalist stock. It won’t be a drop in though.
A smith should be able to mill out the front of the mag opening in the stock and open up the barrel channel to suit the barrelled action.
It might be worth asking B&C if they can do this work for you.
I don't believe the issue is hogging out the barrel channel or inletting the action--it's that B&C doesn't believe it's strong enough for the recoil (probably without a crossbolt).