I have made up a couple of barrels for Type 38 Arisakas (In .308). The Type 38 breeching is very similar to the T99 excepting for the thread pitch. ( The bolt head actually projects into the barrel) It takes a little more machining than other rifles but is not hard for a machinist. The T99 threads are an odd ball pitch and can't be cut on most inch lathes.

Also, It seems like I've heard of people using 7.63 x 39 chamber inserts for this conversion using the original 7.7 barrel. ??
Salt flat