Unlike the big Kz's, the Kz400 only had separate regulator and rectifier with excited field alternator. So separate reg and rec indicate that it is not a permanent magnet alternator.
If the bike has a separate regulator and rectifier, you can use the radio shack rectifier, but there are more powerful ones available, and probably cheaper on Ebay. If you have a 3-phase alternator, you will probably want 3 of those bridge rectifiers. And again, there is probably a cheaper, single 3-phase rectifier available on ebay.
If your regulator and rectifier came as a single unit, then the separate rectifier would be redundant. You would still need the regulator, which would have a rectifier built in.
If your alternator is 3-phase, and originally came with a single reg/rec combo, you can use the one Nessism suggested, or similar ones from modern Ninjas for around $10 or $20.
If your alternator is single phase (two wires) and originally came with a single reg/rec combo, then you will need to find a similar unit. There is a chance you can use a 3-phase unit and leave one of the reg/rec phases unconnected.
Here's the 3-phase version wiring for just the rectifier, but there are probably simpler cheaper options on ebay.:
s3.amazonaws.com/gpzweb/RegRec/GPZdiy3phaseRectifier.html
