[71GMC_3/4T]

Actually, depending on who did the HEI conversion, it could very well still have the resistance wire on it. Mine did.

If that ain't it, I'd be checking things that are hot when the key is in the on position, but not start. This would be things like heater motor, wipers, etc.
Turn the key to on and pull the wire that goes to "batt" on the HEI. Measure the voltage there when "on" and "cranking". The voltage when cranking may drop due to the starter draw, but the voltage should be a full +12V when it's just "on".
Good luck and let us know what you find!