No spark / No start

Need help on a 73 duster, slant six. I am not getting spark at the plugs and a sluggish engine crank. I have replaced battery, starter, starter relay, battery cables, ballists, starter, coil, condenser, distributor, distributor cap, rotor, spark plugs & wires, voltage regulator and igniton control module. coil is giving off spark but not real strong. Carb is getting fuel. what am I missing???