440 problems/quenching?/advice needed

To me listening to the video it sounds like the first place to start is valve train, if the cam is a solid with non adjustable rocker they are definitely tight. To me it sounds "lazy" and unresponsive when loaded. Timing isn't going to do a thing for an improper set up valve train. 516 heads factory had smaller 1.60 exhaust valves and could be choking the air flow. That cam and set up should eat a lot of airflow. To me it sounds like there are a lot of unknowns that need to be verified, and unfortunately that may require diving deeper into it to figure it out. To be zero decked they more than likely cut a lot of material off the decks, most flat top Pistons I've seen without valve reliefs are .080" plus in the hole. With that you get into either cutting the intake or intake side of the head to properly mate the intake. That being said there could be vacuum leaks in that area if it wasn't done properly. Quench in and of its self isn't a bad thing, but there were a boatload of engines that ran like a scalded dog without it being the primary focus. Also things to consider beyond the engine are converter and gears, but I say work the engine first. Just my thoughts.