Resurrecting an old thread. So the answer is yes* it will fit under a stock GT350 hood. And close. However, engine lowering mounts should be used. Not sure which side the hellcat throttle body electronics are on but the GT500 definitely shares the same space as the hard coolant line in the front...
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.
They do have the spike just like the kooks headers i had beside them. They are full 3" and we're waiting on the 3-2.5" conversion connectors to come in the mail from amazon to install them.
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.