'68 383 Barracuda

Just guessing... sounds like it's not getting enough gas at higher revs. Once you let off of it does it start to run again... smoothly? Knowing it's a big block that carb is on the smallish side. I have a 750 double pumper on my small block 360 but it has a good sized cam, Eddy heads with CNC work done to them and doesn't run overly rich. I'm guessing your 600 works fine until you get on it.
Once you get it up in high gear and just nail it to the floor does it stall?