.......I know the question was for a stock 383, just thought I'd throw this in here.
But the OPs question is about blowing it up, not what is the right shift point. So I think you are being appropriately responsive to the OPs question, IMO.
We have no information on what's in his "stock" motor, so no way could we even guess to what the right shift point would be.
If it is a 335 HP 383, factory peak power was 5200, suggesting to me the best shift was probably 5500 to 5700 rpm. If you want to slow your car down, shift at peak power.
