We'll need 'em. And I've got 'em...
Well I've got them around somewhere, but if I can get someone else to do the work of posting them...
Gray Pudding
Climate/Terrain: Subterranean
Frequency: Uncommon
Organization: Solitary
Activity Cycle: Any
Diet: Any
Intelligence: Low (5)
Treasure: Nil
Alignment: Neutral evil
No. Appearing: 1
Armor Class:
Movement: 9
Hit Dice: 10
THAC0: 11
No. of Attacks: 1
Damage/Attack: 4d8
Special Attacks: Surprise, acid
Special Defenses: See below
Magic Resistance: 20%
Size: L-H (8’+ diam.)
Morale: Steady (11)
XP Value: 3,000
Okay, so the main differences vis-á-vis an AD&D black pudding are:
Faster movement (9 vs 6)
Sapiency (Int 5-7 vs Int 0)
More damage (4d8 vs 3d8) due to potent acid
Sneaky (-4 surprise penalty)
Magic Resistance (20%)
Divide when struck by fire or lightning spells
We've got most of that already with our earlier conversion of the Stone Pudding, as I mentioned previously.
Once we agree on how much we increase the acid damage and what bonus to give it on Hide and Move Silently we're effectively done with the stats.
It's stealthier than a Dense Pudding (-4 surprise vs -2), but its lower Int means it'll have fewer skill points. That suggest we'll need some pretty hefty skill bonuses to keep it sneakier.
The original monster did one-third more damage. If we give the "upgraded" version the same boost to the SRD Black Pudding's 2d6+4 plus 2d6 acid (average damage 18) it'll need to do 24 damage on average. i.e. it needs to do about 6 points more acid damage, suggesting 3d8 or 2d12 acid.
As for the sneakiness, I'm still fancying the +8 racial bonus to Move Silently and +4 racial bonus to Hide I suggested earlier.