I saw your VIN in your sig earlier today and quite frankly, it's kinda confused me.
Sounds like someone did away with the series one vent and C pillar bits. Behind the C pillar badge on a series one car, there is nothing, just two holes for the emblem to go on.
One way that should tell for sure is your rear deck. Lift up the carpet. Just behind the seat/ in front of the strut towers should be about 12 holes if it's a series one, or two compartments roughly 6"x10" each if it's a later model.
Here's a pic, although it doesn't show the whole thing...
Neither of those are series one specific. All 240Zs should weigh in around 2300lbs - it is said the earlier ones (70/71) are a little lighter though. Any kind of frame rail, either in the engine compartment, under the floors, etc, will be the same on all 240Zs.