69 340 problems

Do you have good grounds, battery to engine and engine to firewall? It sounds like you`ve covered everything : fuel- yes, air- yes, valve timing- yes, compression- yes, spark- yes but weak. Try pm`ing Moper , he`s great at internet troubleshooting. See if he will read this thread and can give you some insight. There must be something simple but critical that we`ve all overlooked.