Flavor ties and magical beast is ok by me.
Yes, I like Magical Beast over Aberration too, although we can crib some stats from nagas.
So, do we want to keep them Gargantuan? A typical Giant Constrictor Snake is probably about 40 feet long, so I'll argue for reducing them to Huge size, since 50 feet isn't that much longer.
As for ability scores ...
Medium Constrictor Snake: Str 17, Dex 17, Con 13, Int 1, Wis 12, Cha 2
Giant (Huge) Constrictor: Str 25, Dex 17, Con 13, Int 1, Wis 12, Cha 2
Huge Viper: Str 16, Dex 15, Con 13, Int 1, Wis 12, Cha 2
Guardian Naga (Large): Str 21, Dex 14, Con 19, Int 16, Wis 19, Cha 18
Take the Guardian Naga's mental stats and upsize it by one step, bump up Int and Wis by 1 apiece, and increase Dex to 15 (since Huge snakes have better Dexterity than the Dex 12 a standard Huge-sized Guardian Naga would have.)
That would give us:
Serpent Lord: Str 29, Dex 15, Con 23, Int 17, Wis 20, Cha 18