Original load was flat base spitzer 138 grain, so I'd start there, don't get fancy and hope 6.5 boat tailed SMKs will do anything but misbehave - they were my first load because I had them lying about, we all had fun firing them at the range while the rest stood behind trying to guess where they'd impact... (-: My next load try will be flat-based, heavy round noses, if they fail Miss Arisaka 38 will be a safe queen come wallhanger.
The twist will stabilise a heavier, round nose bullet (which was the original military load) and it's more likely to suit your rifle if it is at all generous with its throat or bore. OK so a heavier bullet does more of a rainbow, but how far out do you want to go with open sights? The main problem is accuracy, not trajectory. I read somewhere in a WW2 report that the Japanese rifle bullets often keyholed back then, before hitting stuff, but there was a lot of racist propaganda about. They certainly keyholed on impact and made a mess, no doubt fully intentional.
The actions are very strong so a lot of leeway with the smallish case. I say "smallish" because the 6.5 jap still has 1.5x the muzzle energy of a 5.56x45NATO. It's really the WW1/WW2 Goldilocks cartridge and would be today except for being a long action.
Hogdon has load data, some examples:
Let's know how you get on.
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