Most monsters with the swallow whole ability end up with high CMB totals (due to size and Strength). I am fine the CMB for grappling, but it might be better to shift the swallow whole ability to a Reflex save instead of a grapple check. A fail save would still leave the target grappled.
To illustate my idea, I have converted to SRD creatures, the behir and the tryannosaurus to Pathfinder using the rules in Alpha 2 and my suggestion for a Reflex-based swallow whole ability. Take a look at the write up for both creatures and post your comments.
One thing, I treated improved grab as the Improved Grapple feat for giving a +2 to grapple checks.
BEHIR
CR 8
Often N Huge magical beast
Init +1; Senses darkvision 60 ft., low-light vision, scent, Perception +5
DEFENSE
AC 20, touch 9, flat-footed 19 (–2 size, +1 Dex, +11 natural)
Hp 94 (9d10+45)
Fort +11, Ref +7, Will +5
Immune electricity, cannot be tripped
OFFENSE
Spd 40 ft., climb 15 ft.
Melee bite +15 (2d4+12)
Space 15 ft.; Reach 10 ft.
Special Attacks breath weapon, constrict (2d8+8), improved grab, rake (1d4+4), swallow whole
TACTICS
During Combat A behir usually bites and grabs its prey first, then either swallows or constricts the opponent. It can employ its claws only against foes caught in its coils. If beset by a large number of foes, it uses its breath weapon.
STATISTICS
Abilities Str 26, Dex 13, Con 21, Int 7, Wis 14, Cha 12
Base Atk +9, CBM +19 (+21 grapple)
Feats Cleave, Great Cleave, Power Attack, Skill Focus (Perception), Skill Focus (Survival)
Skills Climb +16*, Stealth +5, Perception +5, Survival +5; *Behirs have a +8 racial bonus on Climb checks and can always choose to take 10 on Climb checks, even if rushed or threatened.
Languages Common
ECOLOGY
Environment warm hills
Organization solitary or pair
Treasure standard
Advancement 10–13 HD (Huge); 14–27 HD (Gargantuan)
Level Adjustment —
SPECIAL ABILITIES
Breath Weapon (Su) 20-foot line, once every 10 rounds, damage 7d6 electricity, Reflex DC 19 half. The save DC is Constitution-based.
Constrict (Ex) A behir deals 2d8+8 points of damage with a successful grapple check. It can make six rake attacks against a grappled foe as well.
Improved Grab (Ex) To use this ability, a behir must hit a creature of any size 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 attempt to constrict the opponent or swallow the opponent in the following round.
Rake (Ex) Six claws, attack bonus +15 melee, damage 1d4+4.
Swallow Whole (Ex) A behir can try to swallow a grabbed Medium or smaller opponent. The target is allowed a Reflex save (DC 19) to avoid this. A failed save leaves the target grappled, though the attempt to swallow does not count as maintaining the grapple. The save is Dexterity-based and the behir has a +4 racial bonus to the DC.
A behir that successfully swallows an opponent can use its Cleave feat to bite and grab another opponent. A swallowed creature takes 2d8+8 points of bludgeoning damage and 8 points of acid damage per round from the behir’s gizzard. Swallowed creatures can attempt to free themselves by inflicting 25 points of damage to the gizzard (AC 15) with a light weapon or unarmed attack. Success results in a gag reflex that immediately expels the swallowed creature.
A behir’s gizzard can hold 2 Medium, 8 Small, 32 Tiny, or 128 Diminutive or smaller opponents.
TYRANNOSAURUS
CR 8
Always N Huge animal
Init +1; Senses low-light vision, scent, Perception +17
DEFENSE
AC 14, touch 9, flat-footed 13 (–2 size, +1 Dex, +5 natural)
Hp 191 (18d8+90)
Fort +16, Ref +12, Will +8
OFFENSE
Spd 40 ft.
Melee bite +20 (3d6+13)/19-20
Space 15 ft.; Reach 10 ft.
Special Attacks improved grab, swallow whole
TACTICS
[Before Combat[/b] A tyrannosaurus is a large ambush predator, using dense foliage to hide its large size.
During Combat A tyrannosaurus pursues and eats just about anything it sees. Its tactics are simple — charge in and bite.
STATISTICS
Abilities Str 28, Dex 12, Con 21, Int 2, Wis 15, Cha 10
Base Atk +13, CBM +24 (+26 grapple, bullrush)
Feats Intimidating Prowess, Improved Bullrush, Improved Critical (bite), Improved Natural Attack (bite), Run, Skill Focus (Perception), Skill Focus (stealth), Skill Focus (Survival), Toughness
Skills Intimidate +9, Stealth +5, Perception +17*, Survival +8; *A tyrannosaurus has a +2 racial bonus to Perception checks.
ECOLOGY
Environment warm plains and forest
Organization solitary or pair
Treasure none
Advancement 19–36 HD (Huge); 37-54 HD (Gargantuan)
Level Adjustment —
SPECIAL ABILITIES
Improved Grab (Ex) To use this ability, a tyrannosaurus must hit an opponent of up to one size smaller 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 try to swallow the foe the following round.
Swallow Whole (Ex) A tyrannosaurus can try to swallow a grabbed opponent of up to two sizes smaller. The target is allowed a Reflex save (DC 20) to avoid this. A failed save leaves the target grappled, though the attempt to swallow does not count as maintaining the grapple. The save is Dexterity-based.
A swallowed creature takes 2d8+8 points of bludgeoning damage and 8 points of acid damage per round from the tyrannosaurus’s gizzard. Swallowed creatures can attempt to free themselves by inflicting 25 points of damage to the gizzard (AC 12). Success results in a gag reflex that immediately expels the swallowed creature.
A Huge tyrannosaurus’s gizzard can hold 2 Medium, 8 Small, 32 Tiny, or 128 Diminutive or smaller opponents.
-A NOTE OF THE TYRANNOSAURUS-
The pathfinder skill and feat systems gave the creature a lot more skills and feats to play with. I tried to match it to the most recent views of this creature as an ambush predator, while staying close to the 3.5 version.