In 1969 (for the N10 exhaust), both the Z28 and L48 had 2.25" head pipes EXCEPT where they attached to the exhaust manifolds. That "connection point" was 2" but I'm not sure how far that 2" pipe extended down until it turned into 2.25". The "over the axle" pipes were also 2.25", but the tail pipes were 2".
For NC8, the Z28 headpipes were 2.25" in diameter EXCEPT where they attached to the exhaust manifolds. Again, that "Connection point" was 2" like the N10, and again, I don't know how far down it extended until it turned into 2.25". The short "over the axle" pipes were 2" after that first chambered section and then the tailpipes were also 2".
Steve, take a look at page 109 in Jerry's "The Definitive 1969 Camaro Z28-SS396 Fact Book" and you'll see a shipper of a April, 1969 Z28 with the NC8 option.
I think Ed covers this....2" at the end connection point (the connection to the exhaust pipe on one side and the heat valve on the other side) that increases to 2 1/4" from about one or two inches below the connection all the way to the muffler.