They probably don't sell prefits chambered in 338lm because they don't want people to be enticed into rebarreling their standard 110 actions and having owners holds them accountable for the failures that may soon follow.
As others have said, "word on the street" is that 338lm specific receivers from savage are stronger than standard long action receivers.
But if you buy the 111 already chambered in 338lm, you're gtg. Just buy a barrel blank and have a Smith thread and chamber it for you.