Sounds like you have already chosen a couple of good cartridges with the 375 H&H and 458 Lott!

If you get a "pair" of Rugers you can just use the standard Ruger rings and not have to worry about bases either.
The original Ruger Rings can be improved by a huge amount by lapping and bedding.
For a scope I would personally choose Leupold 1 1/2 -5 or 1-4 or maybe a little 2 1/2 for the big noisy 458 Lott!
Have Fun