valve float

What motor do you have? What the spec on the cam? What dist/ign box are you using?

On my 340 in my 67 had RPM Performer heads with a Crower solid that was 248/252 and .525/.528 with all MSD stuff and it would sing all the way to 7200-7400 until is just wouldn't pull anymore. I think the springs are good, I'd double check the dist/ign make sure that up to the task.