Re: 243 to 7mm08
Don't worry about the bent necks. It can't be avoided the way you are doing it. Just fireform the brass and it will straighten out. The new Lee expander rods might make this process easier - tapered.
Another easier way is to fireform the cases is using Cream of Wheat. Prime the 243 cases, use about 12gr of pistol powder (HP 38, work up) with Cream of Wheat to the base of the neck, add a small piece of paper towel to keep everything in the case, fire in your rifle. What will emerge is a perfectly formed case. Size as normal. You may want to anneal the necks after you do this if splits start to happen after a few firings.
This method assumes that the 243 cases headspace properly in the 7-08 chamber. If 243 cases are too short, you will need to form a false shoulder in the necks - expand then size down. At which point, your expander die approach will work just as well.