MrFormal: I've written instructions describing how to add "special vehicles" (the Huey and Cobra helicopters, the armadillo, etc.) to the vehicle selection menu:
github.com/CahootsMalone/interstate-76-stuff/blob/master/how-to-use-special-vehicles.md

These instructions include ready-to-use modified vehicle list files for Nitro Pack (as well as modified variant files for the Huey and Cobra helicopters with additional armor).

Note that these files assume that no user-created variants exist (the variant counts for each vehicle are set to their default values), but they can be modified using a hex editor based on the info in this document: github.com/CahootsMalone/interstate-76-stuff/blob/master/file-format-vehicle-list.md

The special vehicles aren't very practical, but it's moderately amusing to try them out. Interestingly, the AH-1 Cobra helicopter (which, unlike the Huey, isn't accessible using a specially-named Rattler variant, meaning only hex editing can make it available to the player) has a fully-modeled interior view.