Since we've wrapped up the "bears", lets tackle
Titanoboa.
This isn't a conversion, per se, but we can easily tie it to any of the giant snakes.
By its listed size, it should start at Gargantuan, and probably advance to Colossal.
We can probably upsize the giant constrictor snake and make some modifications. The dire snake probably won't work, as it combines traits of vipers with constrictors.
Here's a giant constrictor snake upsized to Gargantuan and 25 HD (the mid-range of its usual Gargantuan advancement)....
Snake, Giant Constrictor
Gargantuan Animal
Hit Dice: 25d8+75 (187 hp)
Initiative: +3
Speed: 20 ft. (4 squares), climb 20 ft., swim 20 ft.
Armor Class: 17 (–4 size, +3 Dex, +8 natural), touch 9, flat-footed 14
Base Attack/Grapple: +18/+41
Attack: Bite +25 melee (2d6+16)
Full Attack: Bite +25 melee (2d6+16)
Space/Reach: 20 ft./15 ft.
Special Attacks: Constrict 2d6+16, improved grab
Special Qualities: Scent
Saves: Fort +17, Ref +17, Will +9
Abilities: Str 33, Dex 17, Con 17, Int 1, Wis 12, Cha 2
Skills: 28 (giant constrictor has 8 ranks in Hide, 2 ranks in Climb, Listen and Spot, and 1 in Swim)
Feats: 9 (2 can be epic)[giant constrictor has Alertness, Endurance, Skill Focus (Hide), Toughness]
Environment: Warm forests
Organization: Solitary
Challenge Rating: 9
Treasure: None
Alignment: Always neutral
Advancement: 26–33 HD (Gargantuan); 34+ (Colossal)
Level Adjustment: —
Constrict (Ex): On a successful grapple check, a titanoboa deals 2d6+16 points of damage.
Improved Grab (Ex): To use this ability, a titanoboa must hit with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can constrict.
Skills: Snakes have a +4 racial bonus on Hide, Listen, and Spot checks and a +8 racial bonus on Balance and Climb checks. A snake can always choose to take 10 on a Climb check, even if rushed or threatened. Snakes use either their Strength modifier or Dexterity modifier for Climb checks, whichever is higher. A snake has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.