Personally I would make use of the existing screw holes and modern bases and rings. You obviously don't have the rings to go with the Rigby mounts and there is much more choice if going with modern. No matter what you do, unless welding up the screw holes front and back, the rifle will never be untouched original but is a beauty in its current guise and will serve you well.
I used 44.0grs IMR4064 with 140gr or 45.0gr IMR4320 with 145gr bullets in my 7x57 for velocities 2800 to 2865fps.