You say a "near on 30 year old" Model 70, so I'm assuming that is when it was manufactured, hence not a Classic, introduced in 1994. Unlike the pre-64 models, feeding is determined by the guide lips on the magazine box, not by the rails, so converting from a 7mm Remington Magnum should present no problems. Also, since the magazine length is 3.72", .22" longer than that of the pre-64 Model 70, you should be able to seat bullets out as long as you like.

The .358 Norma Magnum is simply a belted version, with slightly less case capacity, of the .35 Newton, invented by Charles Newton in 1915. Newton claimed 2975 fps. with the 250 grain bullet, which is phenominal performance with the powders then available.