You usually want to cut to length after it has been fireformed, but in this case (with such a large increase in neck diameter required), you'd be best to cut it to about 0.100" of final length before fireforming, in order to remove as much as possible of the neck and shoulder, as they are most likely to split. In this case it's moot, because the 348 brass is shorter than 11mm Gras, but due to the long neck, works fine.
Anneal the top half only of the 348 brass. 15 grs of Unique under cream of wheat and topped with either tightly packed tissue or the wax plug would be a good start. If it is only partially formed after firing, re-anneal and repeat. Once it's to final shape, do a final trimming.
