I'd bet the valves are not closing completely when rpms go above 3500. I use adjustable rockers on every Mopar engine I mess with. I also set the valves at a zero lash and have yet to experience valve float. I have had stock bottom end 360's rev to 7400 rpms with no issues or valve float. I feel that when you push the lifter plunger down you allow the lifters to over pump up when running. My father was a big performance engine builder back in the 60's and 70's and taught me this early on.