Let me clarify, there were special hemi specific 26 lb springs with a shorter install height (I think Hemi stock is .815) and the standard white stripe spring which is a .850 install height. Now he mentioned a big valve head without providing anything about it. I have to make assumptions here, If he got SI stainless valves they come either .050 or .150 longer depending on size, some of the smaller valves you can get stock length. So Lets say he had hemi specific 26lb springs made for a .815 install height to give approx 26lbs of seat pressure and the new head has a .050 or .150 longer valve then the install height will be either .900 (about 20lbs of seat pressure) or 1 inch which is not even in the valve spring chart. Valve float will generally appear at a specific and repeatable RPM if the acceleration of the valve overcomes the spring pressure. It will make a funny noise, engine will get quieter, and not gain any more RPM. There is another kind of valve float caused by valve spring harmonics that can appear then go away and then the engine keeps accelerating again. He needs to measure the installed heightof the spring and that will clear things up.
M