Went through something real similar recently with my son's 350/350 '74 elco..
Same type of issue, it only surfaced when the engine was warm and at a certain rpm range. He was convinced it was bad gas, even though it had been going on for a few tanks worth, but it would run fine cold, and 75-80% of the time.
Ended up being a plug wire with a pinhole (near the plug boot) we didn't even think to check them due to then being less than 6 mo's old and a better quality brand/set, just happened to see it arc while troubleshooting into the evening. New set of wires, swapped just one out, problem solved! (He did go back and swap them all out the next day when he had more time.)