explain my take on it and if ANYONE finds me missleading help me out. I'm just interested in this conversion myself and have done some research.
There are those who say that the 307 and 356 brass are thicker than the 308 and 358 brass and therefore stronger. I don't believe this and measurements have proved this to me. Off the shelf ammo has the 307, 356, and 358 all SAAMI spec'd at 52,000psi. The 308 at 62,000psi. This just tells me that there is no brass problem with this conversion. Internal volumes are the same and the 356 and 358 brass are the same length. However, the coal of the 358 is 2.76" as compared to the 356 which is about 2.5" The case length of the two are essentially the same. Long pointy bullets with good BC's make the cartridge longer.
A gunsmith would not chamber the 358 lever rifle in Marlin to fit the shelf ammo for two reasons. One, he wouldn't want a mistake to happen and let a foolish person feed pointy bullets into a tubular magazine and 2nd, a Marlin can't be made to cycle a cartridge of 2.76" with pointy bullets. I think that if a Marlin receiver is heavily modified a 2.68" coal can be cycled. But that's it as far as I have been told.
So, it's all about the logistics and not danger. 358 is rimless, a handloaders operation, and you must use flat point bullets. But it would be safe and you likely would have the only Marlin 358 in camp. Good luck and take care. realidahorock