The twist on the 6.5 Jap is 1:9 and on the 6.5 Swede it is 1:7.5. I really wouldn't bother rechambering. I would just stock up on the correct brass and shell holders. I found a similar situation with 8mm Sellier and Bellot brass, made to european specs and trying to reload using American shell holders, so I just purchased a whole bunch of Remington brass and I'm good with it. In your case you just need the right shell holder.
Bullets for the 6.5 Jap run at .264 and Hornady and Sierra make good slugs for the bore. Good hunting loads for that bore use 120 - 140 gn bullets and 160 gn rn slugs are available for mil-spec. Remington bulk bullets are as good as anything else. 3031, 4895, 4064, all good powder choices.
The Arisaka is a very strong rifle and you should not have any problems whatsoever with any load. Be safe, and have fun............