Some questions and theories
Does the stumble happen under load or not? If under load it could be the fuel supply leaning out from lack of sufficient fuel pump pressure. Not sure if different carbs would change this or not.
Hydraulic lifters? Is the oil pressure ok? If the pressure is high and the ramps real aggressive on the cam the lifters might be pumping up if the springs aren't correct or weak.
On the first GTO I ever had it would start to stumble at 4500RPM. It was valve springs.
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