Are we talking about the same thing, Stalker? We're looking at vipers, not constrictors; and the Huge viper in the MM has 4 hit dice. (The Large viper has only 3 ...) Now, if we were talking about a druid able to wildshape into Gargantuan creatures (i.e. an epic-level druid), I'd say she could take the shape of a minimum-hit die Gargantuan viper, which would have 7 hit dice and a higher poison save DC.
The druid can pick her size on variably-sized animals, but her abilities will always be based on the physical attributes and hit dice of the lowest-hit-die example of that creature of that size. For example, I don't see any reason that an eighth-level druid couldn't wildshape into a Large wolf, which would have different stats from the base wolf entry in the MM (higher Str and Con, lower Dex, lower size AC adjustment and higher natural armor). But the high-level druid can't wildshape into a '6 hit die' Huge viper to get a couple extra points on that poison DC. And she certainly can't use her own hit-die total to calculate the DC.
[edited for grammar]