If you fireform the brass to fit the chamber, and then only neck size it, the case life may surprise you. What may help with getting the cases fireformed so that they expand concentrically (rather than bulging on one side), is putting a small o-ring around each case the first time you fire it, immediately in front of the extractor groove. Alternatively you could cut a narrow strip (about 1/10" wide) of masking tape and wrap it (a couple of thicknesses) around the case immediately in front of the extractor groove.
The idea is to hold the case concentric with the bore axis, so that it expands evenly. If you neck size, and avoid FL sizing, you save the overworking which tends to be the early death of brass in oversized chambers.
I do this in Lee Enfields I have with generous chambers, where the o-ring (I used to use tape) on the first firing also has the benefit of holding the case hard against the boltface, so that it expands forward to fill the chamber rather than stretching. The same effect should be achievable in an Arisaka if you make the o-ring or tape big enough, or alternatively you could expand the necks and then partially neck size back down to make a sort of false shoulder as far forward as is necessary to hold the case back against the boltface. You only have to do it on the first firing.
