Did this come on all at once?
Was there a slow build up to the pinging?
How much exhaust is coming out of the tail pipe? Is it really hot?
Have you tried pouring a cup of water slowly down the carb with engine at fast idle? This steam cleans the carbon out of the chambers.
I have never heard or seen a HEI module act up, they work or they don't.
Have you verified true top dead center and verified the timing marks are correct?
Have you pulled all eight plugs to see how they compare to each other?
Does the engine temp stay steady? or does it climb really fast?
How does the exhaust sound? normal or quite?
I would lean towards plugged exhaust.
