If you use the Type 38, you can follow what used to be common practice before Arisaka ammo was commercially available, and simply rechamber it to .257 Roberts and handload 6.5 slugs.
The Types 38/99 are the same strength-wise, just as the Springfield 1903 & 03-A3 are - the differences, besides cartridges, are cosmetic.
Just as the 03-A3 production changed to stamped parts from the milled parts of the 1903 Springfield, so also did the Type 99 manufacture change from the way Type 38's were made.
While the Arisaka is Mauser-based, I'd only use one for a "sporter", and not a custom rifle - for which IMHO a Mauser 98 is better suited.
YMMV, of course.
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