Fireforming is a waste of time, money, primers and powder. I've sized hundreds of .30-06 into .35 Whelen just by running them thru the Whelen sizing die. Just be sure the inside of the necks is well lubed or you'll stick a case in the sizer die. Check the newly made .358 cases with a case length gage which also checks the case headspace. You want to be sure the case headspace is correct, neither too little which creates excess headspace in the case nor too much which will not allow the case to chamber. The newly formed cases will also need to be trimmed as the forming operation will not leave them square also they may be a little long. Your gage will show you this.
