If you have a good Smith you could chamber the barrel with a 6mm ARC reamer with a 25 cal pilot and cut the neck and throat separately. PacNor did my 25-204 barrel like that 204 reamer with 25 cal pilot and cut the neck and throat separate.
Or you can have a reamer made. I had a 22-204 reamer made. This was 20 years ago. PT&G and PacNor were great to work with.