I may be completely wrong about this, but it's going to be hard finding pre-1980 Japenese bikes that are of high enough quality to race. I don't believe it was until the early/mid 1980s that Japanese makers finally 'got it' and started making higher-quality, race-worthy frames and components (1st gen dura ace brakes had 'safety' levers).

Anyhow, if you're definitely wanting Japenese steel, the Centurion Ironman bikes seem to be on sale all over the place. It might also be fairly easy to find a nicer Nishiki or Fuji. There's really nothing much to look for in terms of the quality of Japenese frames that you wouldn't look for in any other frames. A lot use Tange tubing, which I don't have any complaints about.