Cutting off the bottom of a 257 Roberts AI neck die is a possibility that I will consider.
Right now I can use my 250 Savage AI neck die and remove the decaping pin and rod. Then do the decapping in a separate operation with the 25-308 AI full length die and the decapping pin and rod extended as far as possible. It's a fiddley process, which I used to create a few dummy rounds to help determine the chamber reamer dimensions (see parallel thread titled "Looking for 25-308 AI Dimensional Drawing").
Looking into John Widden's website, I might go a different route and have a full-length die made, based on fired cases from the actual (future) chamber. That would allow minimum working of the brass, and never having to FL resize with a too-small, standard die after every 2 or 3 neck sizings.
On a happy side note, when creating the above mentioned dummy rounds yesterday, I confirmed I can use a longer cartridge over all length in my Savage M11 action and magazine. Using the longest bullets I have, I can seat the bullets to the point where the boat-tail, if any, is the only thing intruding into the shoulder/body area of the case. This gives the bullets maximum contact with the short case neck, maximum usable powder capacity, and an overall length of 2.85 to 2.92 inches, which fits in my rifle. These bullets (100 gr copper, 110-120 gr lead-core) are probably the longest that will stabilize in my 1:10 twist barrel, since they work fine in my son's 25-308 AI with a 1:10 twist. I used one of them, 117 gr Sierra, when fireforming in my rifle's current chamber -- 250 Savage AI, and they produced sub MOA groups.
The ~130 gr Bergers etc. won't fit so well, but it is unlikely they will stabilize in my barrel anyway.