Does anyone know what the OEM windshield thickness was for the 240Z? 

Just checked the original windshield removed from my 7/70, and it measures 7mm (.275") thick.

Is there some factory documentation that can verify the OEM thickness?

From the US L-Drive fiche, item "9", I assume from the image below "T = 6.0mm" = Thickness = 6.0mm. Same on my R-Drive fiche.

Interesting, mine is definitely closer to 7mm thick, and to the best of my knowledge is original, I've owned the car since 1978. Here are a couple of pics while measuring. First one is lower left corner, 0.267" (6.78mm), near the Nissan OEM marking.  And the second is 0.273" (6.93mm) at the upper edge near center. Possibly 6mm is a minimum tolerance?

Found this topic about windshields, @PrincePaulwhat is the source for the OEM windshield thickness of 6.76mm that you mention in the first post?

 

@CanTechZ Front is 6,76mm and rear 3,85 mm acc. to the glass manufacturer. I measured the following front: 6,52 mm and rear 3,66 mm on my 78" 280Z.

I don´t know exactly where they got that information, but as I´m aware off there where just so many glass thicknesses available and together with the intermediate foil it comes down to the values they supplied.