do you have to change the bolt or action for a new caliber?

I am going to make a custom rem 700. I was going to start with a used gun and strip the barrel and action apart and get a new barrel and so on. I found many cheap used rem 700 in 7mm rem mag but i am going to make mine in a .308. Do I need to get a new bolt for it, and also would that action work for it as well or do i need an original .308 gun?