Lunati Voodoo 383

I have that cam in my 383, recommended by Lunati. .030 over, stock crank, rods, old trw flat tops, eddy heads, street dominator intake, 4.10's and at the time an old fairbanks 2,400 stall. Dart went 12.80's with a bad bog in the carb.
To what rpm does your car pull to before you notice a lag in power? I am thinking i need to change the springs out in mine, seems to drop off to early for the cam. I dont even know what springs are in the engine, was even curious the day i bought the car last year since it has 906 heads on it.