The Viper is indeed designed to the F16 rulebox, meaning the Viper is an F16.
On the other hand, by making it a lot heavier the Viper gets a rating of 104, making it compliant to the 1.04 rules (= having a rating of about 1.04). That rating is important in France at the moment. (They are trying to establish an F104 class).

Bottomline, the Viper is heavier out of a selling point of view.

Regards,
Gill