How hard would it be to change this into 7mm mauser? I found a barrel on midway usa.
About as hard as changing it to .308 since it would require a new bolt, and possibly some other work as well to make it properly feed from the Magazine. 7mm Rem Mag or .300 Win are really the best options for that rifle if you don't want to invest in reloading. What ever you decide to do is going to require you to invest a few hundred dollars to shoot your GGF's old rifle.