Tubing sizes are stated in OD, pipe is ID.
Headers are made of tubing not pipe.
The size of the tubing depends on the application.
2" tubing is about right for a 351, more cubes, bigger tubes.
The size of the tube is related also to where the engines power range is tuned to, as well as the length of the tube and the size of the collector both in diameter and length.
Not to complicate things overly but step tube headers could be a consideration if you are tuning for maximum power.
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