Probably the modern Japanese equivalent of Tange #5 - basic cr-moly that gets the job done. I think a bit of metallurgical ingenuity had to go into those mixte "top tube" bars though.
I have seen very little on the subject, and have read even less.

The Nishiki bikes I've seen new were pretty nice bikes. The brand has a bit of a cult following around here.
Good luck with the build!

-D.S.