Thanks to something posted by dieseldiehard in another thread (
http://www.peachparts.com/shopforum/showthread.php?threadid=79238) I managed to get my tach functioning.
For the record, it turns out the green/black wire from the tach plug at the instrument cluster goes to the #3 prong on the tach amp, and the red/black one just goes to switched power. Everything works like a charm!