I thought about this too, but I rebarreled my 257 weatherby to a 257 weatherby again with an 8 twist barrel.
If you want to shoot 264 cal bullets just chamber it for 264 win mag. It will make your life easier and you won't have to burn rounds down your barrel to fire form 264x257 weatherby.
I literally made my 257 weatherby brass from fire forming 264 win mag brass, so the cases are almost identical in volume. And 264 win mag is just cool. It is the best round that never got the following it deserved.
Its the easiest wildcat around. Your essentially creating a 264 win mag +p with a longer neck.Any information on this round? Thinking of building one.