I'm not sure what you are talking about doing eldeguello but to have a barrel re bored from 7mm to 358 would be an astronomical cost. You would have to completely ream out the bore and rerifle it to boot then chamber it to 358STA.

About the cheapest way to make a 358 STA would be to find yourself a used gun with a magnum length action, (375 H&H length not 300 WM length) or a magnum length action alone,and have a new barrel chamberd for 358 STA fitted to the action. You don't really need to spend the money on a new rifle since you only need the action and the barrel that you will be tossing will have the brunt of the wear.