Let's start Huge but still boost to 18-20 HD. What do you think?
Hmmm, what about Str 27, Dex 12, Con 20, Int 7, Wis 13, Cha 12?
Should we give it a tail slap attack to initiate constrict (like the sea serpents), or transfer to coils after biting like some creatures?
Huge sounds good to me, it's appropriate for a 30-40' serpentine monster.
As for the Hit Dice, I'm a bit divided. If you want to give it True Dragon scale Hit Dice I agree something like 16-20HD seems appropriate.
However, 8 Hit Dice is in the same ballpark as a Giant Constrictor Snake (11HD), a Huge Monstrous Centipede (6HD) or a one-size category downscaled Purple Worm (which would be around 8HD, as a Gargantuan one's 16HD), the much stockier Dragon Turtle is only 12HD and a Dragonne advanced to Huge is only 8HD.
So, I'd be tempted to leave it at 8HD but give it a wide Advancement range, say 9-16HD (Huge), 17-32 (Gargantuan).
I'd be tempted to bump its Dex up to 17, the same as a Giant Constrictor Snake, and drop its Int to 6, the same as a Dragonne. But then, I don't much care for draconic creatures with mediocre dexterity.
I agree with Leopold, Improved grab bite and then Constrict like a snake, plus a breath weapon. The write-up indicates they can constrict one victim in their coils while biting another, I think that'd better carry over into the 3E version.
Now I was wondering, since its magical secretions can cause a whole pool to boil, wouldn't they burn creatures that touch them? If I was writing the thing up from scratch I'd be tempted to have them do additional fire (or cold) damage to anything they grapple/Constrict.