As far as I know, the only place that gets the ej257 is America. There is NO 2.5L sti in japan, that I know for sure. The v7 is the bugeye generation (02-03), which came out here before the sti did, which means there is no such thing as a v7 ej257. The jdm sti motor is the ej207, and has always been called the ej207 for as long as I can remember. v7, v8, v9 jdm motors are ALL called the ej207. In other words, there is no such thing as a jdm ej257. That motor is usdm. The fact that it is single scroll and has tgv housings also attests to that fact. The jdm motor is twinscroll and does not have tgv housings (anymore).
The spec c motor is 2.0L. If that's 2.5L it's not spec c, so it doesn't have a spec c manifold. The spec c manifold is also drive by cable, and that looks like its dbw.
Your best option is to keep the manifold and tgvs from your old motor. That way you don't have to convert to dbw. Other wise you'd need a new ecu, gas pedal, etc.