I don't think it is detonation! Because even if it is detonation when he showers down on the throttle! He should easily pick up speed beyond that rpm range when the throttle is applied slowly. But floating valves can cause a pinging or clicking noise, caused by the lifters lofting off the the cam lobes and when the cam lobes comes around again the lifter hits it and lofts again ( its called float)! the valves are open for the entire revolution of the cam causing no vacuum and no compression (loss of speed, and the engine wont accelerate beyond or near the same rpm each time) until the valve train slows and settles down again and then you might pick up speed to about that rpm again maybe a little more till it starts to floats again! The way Russ explained his problem and just having the engine done with the early 300 valve springs and a different cam then the springs were designed for and probably other then stock lifters and adjustable push rods which could be heavier!! valve loft is a very reasonable assumption especially an an early Chrysler Hemi with a very critical valve geometry!!!
He also said it was built in a shop and blue printed at 10to1 comp so he already knows the octane needed
! And like I said (octane) I don't think is his problem! He already backed off timing, he said it started good,, that must mean hot or cold and he didn't mention any other problems that would point to low octane! But then I could be totally wrong it may be something else