Not being a very popular round there isn't a lot of data for the 7.7x58mm Arisaka, and nearly all of that is IMR powders.
HOWEVER, there are W748 loads for the .303 British, and I think you could use some of that as a starting point.
Do remember that while the Arisaka has a reputation for being a strong action (in blow up tests) it was built for a 45K psi cartridge, not a modern 55-60K psi one. I run my 7.7s at max of 2500fps with 180gr slugs roughtly duplicating original ammo load levels. Do keep an eye on your stock when you go shooting, the Type 99 has a multi piece stock that is glued together and that old glue sometimes comes apart. Had one do that. forearm came apart. No damage to the wood, just "came unglued" during recoil. Cleaned off the old glue and repaired with Elmer's carpenter glue and its been just fine ever since.
IMR 4895 is what I use for the 7,7mm, works well, and I have it...