I will readily admit that I am not really knowledgeable about the Mod 70 Winchester dimensions -- can a 458 simply be converted to a Lott by reaming the chamber or is there more involved?

Depends on which M70.

The pre-64 is a standard lenth action which will have to be opened-up.

I believe the post-64 push-feeds are .375 H&H length actions.

The M70 Classic is .375 length so should be able to ream chamber and remove magazine block.

I have a post 64 model 70 in .458WM. I had thought about converting dine to a Lott, but the magazine box is too short. To do it you would have to replace the magazine, and probably do a bit of stock work. I decided the best way to go was just to buy another gun.