What is happening is you are running out of camshaft. Here are the specs on that cam.
Operating Range: 2500-6500 RPM
Duration Advertised: 280° Intake / 286° Exhaust
Duration @ .050'' Lift: 242° Intake / 248° Exhaust
Valve Lift w/1.5 Rockers: .570'' Intake / .576'' Exhaust
Valve Setting: .016'' Intake / .018'' Exhaust
Lobe Separation Angle: 110°
With the small duration and 110° LSA and a 4.56gear w/28" tall tire a 383 has it's tongue hanging out by a 1000'. My motor does the same thing because my cam only has 248°/252° duration, 383, 4.88 w/33" tire. JMO.
Curtis