Charlie Im a chevy man for 30 years and I may be able to help you chevy alternators average voltage is 16.5 .Split fire plugs are fine .HEI ign fine.Gas weak a little .600 carb fine may need adjusting! Try taking off distributor cap rotor cap and look at little flippers at top of distributor shaft make sure they move also make sure the springs are there. You can change the weight of the flippers and the size of the springs I like 262 for weight and a heavier spring.You can buy kits at your local car shop.Hope this will help and check timing.Good luck.
I am unsure but I seem to remember that the balast Resistor should drop the coil supply voltage to 9v. Too much voltage to the coil will eventualy cause it to fail. Not sure if this applies to the HEI though. I would check for vacume leaks especialy in the vacume advance. This could be part of your trouble. Also have the altenator checked at the parts store. It sounds like you may have a diode out in the darn thing.
I would put a ground strap from the body to the engine to insure proper grounding to the interior, but the knocking sounds like a timing problem.Has the weather took a drastic change in the past few days? Could be that the temp change needs to be compensated for.
Also check the color of your sparkplugs more info at http://www.fiatparts.com/colorplg.htm