I have seen this problem before, it turned out that the guy that installed the engine in this particular car actually pinched the tachometer wire, grounding it and killing the module pulses. I have also seen where the ground "tang" was left out when coil was changed.
Here is a good thread for troubleshooting the HEI
http://chevythunder.com/hei_troubles...ge_2_coil_.htm
Hope this helps!
JD