With any of these actions, which take true shouldered pre-fits, you do not set the headspace in any way. Savage style nutted barrels require a gauge to SET the headspace by screwing the barrel into the action until it touches the gauge, then locking in place with the barrel nut, but the Defiance Ruckus & Tenacity, Lone Peak Fusion, Impact 737R, Bighorn TL3, Bat Machine, Seekins Havak, American Rifle Company Nucleus, Coup de Grace, Archimedes, etc, Kelby Nanook, Atlas, & Prometheus, Aero Solus, and I'm sure a few others I'm forgetting all take shouldered prefits, meaning the bolt and actions are machined within tolerances such the barrels can be made anywhere in the world and thread up perfectly to the action within tolerance for chamber headspace. So there's no way for you to set the headspace - just screw it in like a lightbulb, torque to spec, and party on.
Using a gunsmith to make your barrels may be more reliable than buying a barrel off of the shelf from some bulk barrelsmith - for example, I have barrels for my Seekins Havak made by a local smith, even though I could order pre-fits online for that rifle, same guy who makes my barrels for a couple other firearms, but I order Proof Research pre-fits for my Defiance Ruckus. I know the two dudes who spin barrels for me will be more likely to yield smaller groups than the barrels I get from the larger commercial gunsmiths, but I also know the few commercial barrelsmiths I've used can all turn out really, really small groups too. I know exactly what reamer my smith is going to use for my barrels, so I get the same exact chamber every time - down to the 1/2 thou - but I can't expect that consistency from the commercial barrel houses. I know I will spend a bit more to get a blank and have it turned by my precision smiths too, but I can text them, drive to their house for a beer, pick my barrels up at a match, drop off actions at their shop, whatever I need, and I know exactly who is running the lathe... So it's really up to you as the consumer to pick which you prefer.
But in 2024 and beyond, it's absolutely foolish to build a rifle on any action which doesn't come headspace controlled to accept pre-fit barrels. Give yourself the options, and don't settle for less.
If I were building a hunting + long range rifle, I'd look hard at the Impact NBK, the Defiance AnTi, and the Lone Peak Fusion Titanium, and I'd probably personally end in a coin flip between the NBK and the Fusion Ti.
One other thing to consider - if you're building a chassis rifle which takes prefit barrels - you have the LUXURY of choosing to have chassis weights and a heavy steel barrel for an ideal cartridge for shooting 1,000 yards, then have a carbon fiber barrel and remove the chassis weights, maybe even change bolt heads to utilize a magnum bolt face to take the same scope, action, trigger, and chassis on any hunt you could imagine. Shoot a 6 Dasher for 1,000yrd steel banging, then when you decide to go afield after elk, cut 10lbs off of the rifle and swap to a 6.5 PRC and go up mountain with fire in your fists. Almost like having 2 rifles in one, with little to no compromise in performance between them for their respective applications.