I personally think that rear mount 70mm turbo is your problem. I had a 6.0 with a 224 cam and the master power t76 up front with the vic jr intake and it ran like a raped ape. Look up 02silvz71 on youtube and you will see the vids. Mine was without meth. its either the too small turbo or your tune.

 

Thanks for all the input guys throwing the front mount manifold on this weekend and my old truck intake on let's see what happens