I have the same style barrel on my Highwall and have been researching this for quite sometime.
Understand that the receivers for both size shanks were the same, just the shank was different.
I've been told on more than one occasion that the biggest I could comfortably go with the small shank would the 38-55.
I have also been told, though it would be a bit more expensive, that the receiver can be opened up to accept a large shank barrel thread and then the sky's the limit. Since the external receiver dimensions were the same no matter which shank was used the amount of steel around the chamber would be the same, (as log as we're talking thinsides, the flat sided ones were a whole different animal) just that the proportion of barrel steel to receiver steel would be different. And to my way of thinking, since modern barrel steels are stronger it would actually be stronger than an original.
Then again my way of thinking is strictly from an amateurs POV, others here can tell me if I'm right or wrong.
I did all this research a few years back and may still have some names if you're interested.
