Have you measured top to bottom or side to side?
I have twos sets not mounted on the car and another one mounted that I can't measure. To me it looks like the inside diameter of the tubing is about 38mm. However, there is some crude porting done that is very visible on my NOS set ... and indeed 1 and 2 have the 40mm dimension top to bottom on both sets. the side to side dimension on the used set is consistently 38mm and varies from 37 to 38mm on the other set. My money is that the apparently different size primaries are attributable to the porting of the flanges rather than due to differnt pipe diameters.
IMO 38mm works better on the street than 40mm or above and it may significantly lower the rpm when max torque comes in.
Re: some of the comments on drivability ... putting 1600 GTA headers with a 2 1/4" GTA exhaust on an otherwise very stock Giulia Super made a huge difference in bottom-end torque and drivability ... right up there with with close ratio box, lightweight flywheel and good shocks ...
Shankle/Centerline headers (38mm) - not the replica GTA type which is 40mm - work great on a 2-liter, especially when combined with an improved merge collector and a good exhaust with 2 1/4 or 2 1/2 diameter. However a ready made Alfaholics set-up may be a lot easier ...
Here are the primary diameters of my GTA headers. This might explain why Classic Alfa offered "big tube" 40mm headers (presumably from CSC). When I get a chance I'll measure my Alfaholics headers for comparison.
#1 40.08 mm
#2 40.11 mm
#3 37.00 mm
#4 37.00 mm