Re: LSA-65 action for .257 Weatherby Mag???
Hand-loaded .257Wby. ammo will be around 3.3" in length. Is your existing magazine-box long enough?
Of course, the bolt-face will need to be opened-up for the larger rim diameter.
I'm not an expert on changing calibers, but have you considered the 25-06 cartridge? It's slightly slower than the Wby., but the critters will not know the difference. And you can still use all those '06 cases.