My thought would be go with the M70 for a .375 and with the Granite Mountain for a .404. I think trying to make a .404 work in a Model 70 is too much of a stretch. The difference in diameter of the cases at the base (.544" vs. .513") would limit magazine capacity and might cause feeding problems.

The barrel shank diameter of a Model 70 is also a consideration. You don't want to get the chamber walls too thin. The Model 70 shank diameter is .100" less than that of a 98 Mauser (and .125" less than a P14 or 1917 Enfield.)

The case length is about the same and the .404 OAL is actually shorter. Nonetheless...