Get the rifle you want in 264 Rem Mag and have it rechambered. Very little other work required. Maybe the long action magazine will have a spacer which can be removed or the box changed. Maybe bolt stop will need to be adjusted. Maybe the feed rails will require a tiny bit of filing. Easy peasy.
I purchased the reamers gauges and my smith charged me $50 to pull the barrel, chamber and head space.
If you want a 28-Nosler, get a 7mm Rem Mag and have it rechambered. Very little other work required.
If you ant a 338/26-Nosler or 33-Nosler, get a 338 Win Mag and have it rechambered. Very little other work required.
If you want a 277/26-Nosler or 27-Nosler or 308/26-Nosler or 30-Nosler you can start with a 270 or 308 or 30-06 but you will need to do more about the bolt face and feeding.
Or start with a magnum bolt face long action and an new barrel. If you were to start the project with a Remington 700 in any of the RUM chamberings, just a barrel change.
If you were to start with a Weatherby Mark V in any Weatherby or magnum magnum chambering. There will be very little work to do.
I've made 4 XXX/26-Nosler. 277, 284/7mm, 308 and 338. All were rechambered barrels. No setback required. All except the 277 required no adjustment to the feed rails. 2 were made from Weatherby's and 2 from R700s. I prefer Weatherby so the R700 ones are up for sale. I am working on the 277/26-Nosler feed rails slowly. As my machining mentor said "you can take more off but you can't put more on moron".