to go from 375 h&h to 416 rem mag...
is this stricly a rebarrel job on the m70?

Yes. Have the gunsmith check the feeding while it's there, but there shouldn't be any parts difference between the 375 and 416. It should just be a barrel swap.

Yes, pretty much just a barrel change.

Have you shot a .416 Rem Mag yet? If not, I recommend doing so before you spend money making the change. I have both, and there is a world of difference between their recoil levels.