In that case we could just tidy up the advancement, give it a length & weight, and call it done. 26-33 HD (Gargantuan); 34-75 HD (Colossal) would fit the usual "triple max HD" scheme, but 75 HD seems too high for a 'realistic' pseudo-prehistoric beastie. My gut's telling me to go double max HD instead, giving us:
Advancement: 26-33 HD (Gargantuan); 34-50 HD (Colossal)
As for length & weight. If a titanoboa is 40' long and 2500 pounds, a double-size titanoboa would be 80' and 20000 pounds. That's still within the standard weight range for a Huge creature (4000-32000 pounds) but, as previously mentioned, snakes in 3E D&D are probably underweight for their size category, just as Oozes are usually overweight, so I see nothing wrong with that.
Now, are we going to tweak its Str and/or Con up a bit or just leave it as is? I am fine with leaving it at Str 33 and Con 17 from a straight size-increase of a giant constrictor.
To recap the changes so far relative to Shade's rough stats for the snake in post
#402:
Snake, Giant Titanoboa
Armor Class: 21 (–4 size, +3 Dex, +12 natural), touch 9, flat-footed 18
Attack: Bite +26 melee (2d6+16)
Full Attack: Bite +26 melee (2d6+16)
Special Attacks: Constrict 2d6+16, improved grab, swallow whole
Saves: Fort +17, Ref +17, Will +13
Skills: Climb +19, Hide +25, Listen +11, Spot +14, Swim +19
Feats: Alertness, Blind-Fight, Combat Reflexes, Cleave, Power Attack, Skill Focus (spot), Weapon Focus (bite)
Epic Feats: Epic Will, Epic Skill Focus (hide)
Swallow Whole (Ex): A giant titanoboa can try to swallow a grabbed opponent of a smaller size than itself by making a successful grapple check. Once inside, the opponent takes 2d8+11 points of crushing damage plus 8 points of acid damage per round from the snake’s gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 25 points of damage to the gizzard (AC 16). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out. A Gargantuan snake’s interior can hold 2 Large, 8 Medium, 32 Small, 128 Tiny, or 512 Diminutive or smaller opponents.
[
I've modified the gizzard damage and AC to account for the changes in Str and natural armour.]