stevex,
If you are left handed it might be easier to rebarrel for a caliber of your choice rather that finding a left handed action chambered in a caliber you wanted to shoot. The rifle you have is a magnum - the bolt face is too large for any but the belted magnum cartridges - if you change to a standard cartridge you will have to replace the bolt.
Without knowing what you hunt or the kind of shooting that you do it is hard to recommend a cartridge but the least powerful magnum cartridge is the 350 Reminton Magnum. You could download this to shoot pistol bullets or load it with 180 to 250 grain bullets for larger game. The velocities are not much higher than those produced by the 358 Winchester. If you need a smaller bore, then the 257 Weatherby would give you a long range 25 caliber round with only a barrel change.
PaulS