Integrate.
Fix the railroad.
What are the advantages/drawbacks to each of the new guns?
Nambu-Garand Type 10: A very reliable, easy to use semi-automatic design, it allows much improved rates of fire for infantry, while maintaining the same range, accuracy and handling of a rifle. The main reasons not to take it is the time it would take to retool factories and the potential mistrust of a non-japanese design, but the existing stocks of older, existing guns might assuage that first problem.
Arisaka Type 96: It's essentially just an update/upgrade to the old Type 38, with a more potent round and some updated ergonomics. It would be less of a departure from tradition, and require limited amounts of retraining. However, it would still require factory retooling and the same amount of resources as producing the Nambu-Garand. Plus, it's still just a Bolt-Action.
Arisaka Type 38: Currently in production and use by most of the military. The issue is just that it's old, a bit antiquated, and most of the military, which is clamoring for an update, would be a bit angry at being told that they can't get a new service rifle.