Cam is not to big. It fact, its a baby.
Carb is not to big. Just needs tuning.
67-74 Q-jets where about 760-780 cfm.
75 or 76-79, they used 800. Even on 350 ci.
Either the jets are to big, AND/or its out of adjustment.
Try'n find my old 67-400(w/744 cam) timing specks tonight.
From memory the closed chamber heads need more lead time.
Or other way around? Been since 82.

Again.
Pull all the plugs and do a cranking compression test.