It doesn't make much difference if they are sized to .358 or not, if one or more of your chamber throats are sized to .357. The bullets will be swaged down to .357. I would not shoot any plain based bullet, especially a light bullet, in 357 with a 2400 load WITHOUT a gas check. To make things worse, Oregon trail bullets are bevel base. You can try it, but without using a gas check you are probably going to get some heavy leading within a dozen or so rounds.
Been there and done that.