New Fuel Tank, but Cap too small??

[TexasLS1]

Just installed a Spectra Fuel Tank #GM49B in my 72 C10 with EEC. The fit is great, and the part number is correct for the truck(unless in really missing something). *HOWEVER* the neck is about 1/4" larger diameter, so my fuel cap from the original tank is too small and drops loosely into the neck. Replacement caps at the parts stores are the same diameter as the original.

***Does anyone know of a cap that the cam/neck is 1-1/2" instead of the standard 1-5/16" cap?

***Is the cap for a 67-70 larger than the 71-72?

***Will there be negative effects by using a vented cap if I find one, since my cap is supposed to be non-vented for the EEC system??

Hoping I don't have to take the tank back out; I shoulda checked it before starting the job but I know lots of guys use the Spectra...