no welding is required but you will need 3"-2.25" if you're using the stock resonator. Unless the 2018 uses a different size mid-pipe. If it was maybe 2 inches longer it would be perfect but it will mate up.
I have a set of the 2m Fab headers. I'm going to install them on my friend's 2015 5.0. the welds look pretty good and it all sides together very well. I'll update after install. It's 1 7/8 to fill 3" so it'll need a transition piece to go to stock mid pipe or a full 3" cat back.