How about a rebore/rechamber of a 416 rather than a total rebarrel?
I could see doing this with a Ruger since they tend to be heavy in the barrel - at least some are, don't remember if the older ones or newer ones had a heavier barrel but I thought it was one or the other.
I could see this as the least expensive route to finding out if the 450 is all you think it could be.